diff --git a/src/action.ml b/src/action.ml
index 757a3e44..2539155a 100644
--- a/src/action.ml
+++ b/src/action.ml
@@ -36,8 +36,7 @@ let expand_prog ctx ~dir ~f template =
     else
       match Context.which ctx s with
       | Some p -> p
-      | None ->
-        die "@{<error>Error@}: Program %s not found in PATH (context: %s)" s ctx.name
+      | None -> Utils.program_not_found ~context:ctx.name s
   in
   match String_with_vars.just_a_var template with
   | None -> resolve (expand_str ~dir ~f template)
diff --git a/src/context.ml b/src/context.ml
index ac21eaa2..f65f366f 100644
--- a/src/context.ml
+++ b/src/context.ml
@@ -44,9 +44,10 @@ type t =
   ; version                 : string
   ; stdlib_dir              : Path.t
   ; ccomp_type              : string
-  ; bytecomp_c_compiler     : string
+  ; c_compiler              : string
+  ; ocamlc_cflags           : string
+  ; ocamlopt_cflags         : string
   ; bytecomp_c_libraries    : string
-  ; native_c_compiler       : string
   ; native_c_libraries      : string
   ; native_pack_linker      : string
   ; ranlib                  : string
@@ -107,7 +108,7 @@ let compare a b = compare a.name b.name
 let get_arch_sixtyfour stdlib_dir =
   let config_h = Path.relative stdlib_dir "caml/config.h" in
   List.exists (lines_of_file (Path.to_string config_h)) ~f:(fun line ->
-    match String.split_words line with
+    match String.extract_blank_separated_words line with
     | ["#define"; "ARCH_SIXTYFOUR"] -> true
     | _ -> false)
 
@@ -168,7 +169,7 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin ~use_findli
      Hashtbl.add opam_var_cache ~key:"root" ~data:root
    | Default -> ());
   let prog_not_found_in_path prog =
-    die "Program %s not found in PATH (context: %s)" prog name
+    Utils.program_not_found prog ~context:name
   in
   let which_cache = Hashtbl.create 128 in
   let which x = which ~cache:which_cache ~path x in
@@ -232,8 +233,9 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin ~use_findli
     | Error (key, _, _) ->
       die "variable %S present twice in the output of `%s`" key ocamlc_config_cmd
   in
+  let get_opt var = String_map.find var ocamlc_config in
   let get ?default var =
-    match String_map.find var ocamlc_config with
+    match get_opt var with
     | Some s -> s
     | None ->
       match default with
@@ -268,6 +270,29 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin ~use_findli
     else
       env,env_extra
   in
+  let c_compiler, ocamlc_cflags, ocamlopt_cflags =
+    match get_opt "c_compiler" with
+    | Some c_compiler -> (* >= 4.06 *)
+      (c_compiler, get "ocamlc_cflags", get "ocamlopt_cflags")
+    | None ->
+      let split_prog s =
+        let len = String.length s in
+        let rec loop i =
+          if i = len then
+            (s, "")
+          else
+            match s.[i] with
+            | ' ' | '\t' ->
+              (String.sub s ~pos:0 ~len:i,
+               String.sub s ~pos:i ~len:(len - i))
+            | _ -> loop (i + 1)
+        in
+        loop 0
+      in
+      let c_compiler, ocamlc_cflags = split_prog (get "bytecomp_c_compiler") in
+      let _, ocamlopt_cflags = split_prog (get "native_c_compiler") in
+      (c_compiler, ocamlc_cflags, ocamlopt_cflags)
+  in
   return
     { name
     ; kind
@@ -299,9 +324,10 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin ~use_findli
     ; ocamlc_config = String_map.bindings ocamlc_config
     ; version
     ; ccomp_type              = get       "ccomp_type"
-    ; bytecomp_c_compiler     = get       "bytecomp_c_compiler"
+    ; c_compiler
+    ; ocamlc_cflags
+    ; ocamlopt_cflags
     ; bytecomp_c_libraries    = get       "bytecomp_c_libraries"
-    ; native_c_compiler       = get       "native_c_compiler"
     ; native_c_libraries      = get       "native_c_libraries"
     ; native_pack_linker      = get       "native_pack_linker"
     ; ranlib                  = get       "ranlib"
@@ -354,7 +380,7 @@ let default ?(merlin=true) ?(use_findlib=true) () =
 
 let create_for_opam ?root ~switch ~name ?(merlin=false) () =
   match Bin.opam with
-  | None -> die "Program opam not found in PATH"
+  | None -> Utils.program_not_found "opam"
   | Some fn ->
     (match root with
      | Some root -> return root
diff --git a/src/context.mli b/src/context.mli
index c16bba51..da6a5809 100644
--- a/src/context.mli
+++ b/src/context.mli
@@ -81,9 +81,10 @@ type t =
   ; version                 : string
   ; stdlib_dir              : Path.t
   ; ccomp_type              : string
-  ; bytecomp_c_compiler     : string
+  ; c_compiler              : string
+  ; ocamlc_cflags           : string
+  ; ocamlopt_cflags         : string
   ; bytecomp_c_libraries    : string
-  ; native_c_compiler       : string
   ; native_c_libraries      : string
   ; native_pack_linker      : string
   ; ranlib                  : string
diff --git a/src/findlib.ml b/src/findlib.ml
index 3f5325e7..c301d6c1 100644
--- a/src/findlib.ml
+++ b/src/findlib.ml
@@ -115,7 +115,7 @@ module Vars = struct
     | None -> ""
     | Some rules -> Rules.interpret rules ~preds
 
-  let get_words t var preds = String.split_words (get t var preds)
+  let get_words t var preds = String.extract_comma_space_separated_words (get t var preds)
 end
 
 type package =
diff --git a/src/gen_rules.ml b/src/gen_rules.ml
index d526f7af..70521108 100644
--- a/src/gen_rules.ml
+++ b/src/gen_rules.ml
@@ -334,22 +334,14 @@ module Gen(P : Params) = struct
      | User variables                                                  |
      +-----------------------------------------------------------------+ *)
 
-  let cxx_compiler, cxx_flags =
-    match String.split_words ctx.bytecomp_c_compiler with
-    | [] -> assert false
-    | prog :: flags ->
-      let comp =
-        if Filename.is_relative prog then
-          match Bin.which prog with
-          | None -> Path.of_string "g++"
-          | Some p -> p
-        else
-          Path.of_string prog
-      in
-      let flags =
-        List.filter flags ~f:(fun s -> not (String.is_prefix s ~prefix:"-std="))
-      in
-      (comp, flags)
+  let cxx_flags =
+    String.extract_blank_separated_words ctx.ocamlc_cflags
+    |> List.filter ~f:(fun s -> not (String.is_prefix s ~prefix:"-std="))
+
+  let cxx_compiler =
+    lazy (match Context.which ctx ctx.c_compiler with
+      | Some path -> Build.Prog_spec.Dep path
+      | None -> Dyn (fun _ -> Utils.program_not_found ctx.c_compiler))
 
   (* Expand some $-vars within action strings of rules defined in jbuild files *)
   let dollar_var_map =
@@ -364,10 +356,11 @@ module Gen(P : Params) = struct
       | Some p -> Path.to_string p
     in
     [ "-verbose"       , "" (*"-verbose";*)
-    ; "CPP"            , ctx.bytecomp_c_compiler ^ " -E"
-    ; "PA_CPP"         , ctx.bytecomp_c_compiler ^ " -undef -traditional -x c -E"
-    ; "CC"             , ctx.bytecomp_c_compiler
-    ; "CXX"            , String.concat ~sep:" " (Path.to_string cxx_compiler :: cxx_flags)
+    ; "CPP"            , sprintf "%s %s -E" ctx.c_compiler ctx.ocamlc_cflags
+    ; "PA_CPP"         , sprintf "%s %s -undef -traditional -x c -E" ctx.c_compiler
+                           ctx.ocamlc_cflags
+    ; "CC"             , sprintf "%s %s" ctx.c_compiler ctx.ocamlc_cflags
+    ; "CXX"            , String.concat ~sep:" " (ctx.c_compiler :: cxx_flags)
     ; "ocaml_bin"      , Path.to_string ctx.ocaml_bin
     ; "OCAML"          , Path.to_string ctx.ocaml
     ; "OCAMLC"         , Path.to_string ctx.ocamlc
@@ -454,8 +447,8 @@ module Gen(P : Params) = struct
           String.capitalize_ascii module_basename
         in
         let deps =
-          String.split_words (String.sub line ~pos:(i + 1)
-                                ~len:(String.length line - (i + 1)))
+          String.extract_blank_separated_words (String.sub line ~pos:(i + 1)
+                                                  ~len:(String.length line - (i + 1)))
           |> List.filter ~f:(fun m -> m <> unit && String_map.mem m modules)
         in
         let deps =
@@ -1327,7 +1320,7 @@ module Gen(P : Params) = struct
          (* We have to execute the rule in the library directory as the .o is produced in
             the current directory *)
          ~dir
-         (Dep cxx_compiler)
+         (Lazy.force cxx_compiler)
          [ S [A "-I"; Path ctx.stdlib_dir]
          ; expand_includes ~dir lib.includes
          ; As cxx_flags
@@ -1858,7 +1851,8 @@ module Gen(P : Params) = struct
            List.iter template ~f:(fun s ->
              if String.is_prefix s ~prefix:"#" then
                match
-                 String.split_words (String.sub s ~pos:1 ~len:(String.length s - 1))
+                 String.extract_blank_separated_words
+                   (String.sub s ~pos:1 ~len:(String.length s - 1))
                with
                | ["JBUILDER_GEN"] -> Format.fprintf ppf "%a@," Meta.pp meta.entries
                | _ -> Format.fprintf ppf "%s@," s
diff --git a/src/import.ml b/src/import.ml
index 1d74cd5b..22377d94 100644
--- a/src/import.ml
+++ b/src/import.ml
@@ -205,24 +205,34 @@ module String = struct
     let uncapitalize_ascii = String.uncapitalize
   end
 
-  let split_words s =
+  let extract_words s ~is_word_char =
     let rec skip_blanks i =
       if i = length s then
         []
+      else if is_word_char s.[i] then
+        parse_word i (i + 1)
       else
-        match s.[i] with
-        | ',' | ' ' | '\t' | '\n' -> skip_blanks (i + 1)
-        | _ -> parse_word i (i + 1)
+        skip_blanks (i + 1)
     and parse_word i j =
       if j = length s then
         [sub s ~pos:i ~len:(j - i)]
+      else if is_word_char s.[j] then
+        parse_word i (j + 1)
       else
-        match s.[j] with
-        | ',' | ' ' | '\t' | '\n' -> sub s ~pos:i ~len:(j - i) :: skip_blanks (j + 1)
-        | _ -> parse_word i (j + 1)
+        sub s ~pos:i ~len:(j - i) :: skip_blanks (j + 1)
     in
     skip_blanks 0
 
+  let extract_comma_space_separated_words s =
+    extract_words s ~is_word_char:(function
+      | ',' | ' ' | '\t' | '\n' -> false
+      | _ -> true)
+
+  let extract_blank_separated_words s =
+    extract_words s ~is_word_char:(function
+      | ' ' | '\t' -> false
+      | _ -> true)
+
   let lsplit2 s ~on =
     match index s on with
     | exception Not_found -> None
diff --git a/src/utils.ml b/src/utils.ml
index 7641f26d..9739c3de 100644
--- a/src/utils.ml
+++ b/src/utils.ml
@@ -79,3 +79,9 @@ let describe_target fn =
     sprintf "alias %s" (Path.to_string dir)
   | _ ->
     Path.to_string fn
+
+let program_not_found ?context prog =
+  die "@{<error>Error@}: Program %s not found in PATH%s" prog
+    (match context with
+     | None -> ""
+     | Some name -> sprintf " (context: %s)" name)
diff --git a/src/utils.mli b/src/utils.mli
index d37dbb35..515b2a30 100644
--- a/src/utils.mli
+++ b/src/utils.mli
@@ -17,3 +17,6 @@ val jbuild_name_in : dir:Path.t -> string
 
 (** Nice description of a target *)
 val describe_target : Path.t -> string
+
+(** Raise an error about a program not found in the PATH *)
+val program_not_found : ?context:string -> string -> _