Fix preprocessing action in src/dune (#1200)

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

src/configurator/dune

@@ -4,5 +4,4 @@
  (name        configurator)
  (public_name dune.configurator)
  (libraries   stdune ocaml_config dsexp)
- (flags       (:standard -safe-string (:include flags/flags.sexp)))
- (preprocess  no_preprocessing))
+ (flags       (:standard -safe-string (:include flags/flags.sexp))))

src/dune

@@ -10,7 +10,8 @@
               ocaml_config
               which_program)
  (synopsis    "Internal Dune library, do not use!")
- (preprocess  (action (run src/let-syntax/pp.exe %{input-file}))))
+ (preprocess  (action
+               (run %{project_root}/src/let-syntax/pp.exe %{input-file}))))
 
 (ocamllex meta_lexer glob_lexer dune_lexer)