Expose a few more variables

doc/jbuild.rst

@@ -692,6 +692,10 @@ Jbuilder supports the following variables:
 -  ``ARCH_SIXTYFOUR`` is ``true`` if using a compiler targeting a
    64 bit architecture and ``false`` otherwise
 -  ``null`` is ``/dev/null`` on Unix or ``nul`` on Windows
+- ``ext_obj``, ``ext_asm``, ``ext_lib``, ``ext_dll`` and ``ext_exe``
+   are the file extension used for various artifacts
+- ``bytecomp_c_libraries`` and ``native_c_libraries`` are list of C libraries
+  used by the OCaml runtimes
 
 In addition, ``(action ...)`` fields support the following special variables:
 

src/super_context.ml

@@ -159,6 +159,13 @@ let create
     ; "ARCH_SIXTYFOUR" , string (string_of_bool context.arch_sixtyfour)
     ; "MAKE"           , make
     ; "null"           , string (Path.to_string Config.dev_null)
+    ; "ext_obj"        , string context.ext_obj
+    ; "ext_asm"        , string context.ext_asm
+    ; "ext_lib"        , string context.ext_lib
+    ; "ext_dll"        , string context.ext_dll
+    ; "ext_exe"        , string context.ext_exe
+    ; "bytecomp_c_libraries", strings context.bytecomp_c_libraries
+    ; "native_c_libraries"  , strings context.bytecomp_c_libraries
     ]
     |> String_map.of_list
     |> function