This requires the development version of Menhir, to be released soon.
In summary:

handcrafted.messages is new.
  It contains a mapping of erroneous sentences to error messages,
  together with a lot of comments.

Makefile.extr is new.
  It contains a rule to generate cparser/pre_parser_messages.ml
  based on this mapping.

cparser/ErrorReports.{ml,mli} are new.
  They construct syntax error messages, based on the compiled mapping.

cparser/Lexer.mll is modified.
  The last two tokens that have been read are stored in a buffer.
  ErrorReports is called to construct a syntax error message.

cparser/GNUmakefile is new.
  It offers several commands for working on the pre-parser.

cparser/deLexer.ml is new.
  It is a script (it is not linked into CompCert).
  It translates the symbolic name of a token to an example of this
  token in concrete C syntax.
  It is used by [make -C cparser concrete] to produce the .c files
  in tests/generated/.

cparser/tests/generated/Makefile is new.
  It runs ccomp, clang and gcc on each of the generated C files,
  so as to allow a comparison of the error messages.