MPR#5107: ocamlbuild misses local C #includes

[damiendoligez]

PR transferred from https://caml.inria.fr/mantis/view.php?id=5107
[original reporter: Camarade_Tux]

When a .c or .h file uses #include "foo.h", ocamlbuild has to copy that file in the _build directory but currently it doesn't, making the build fail.

The attached patch (against today's svn) uses cpp to get these dependencies for .c and .h files: .c files are "allowed" to include .c and .h files while .h files can only include .h files.
The reason for this limitation is that the whole list would be: c, cpp, cxx, cp, C, h, hpp, hxx, hxp, H, CPP, HPP, c++, h++, tcc, hh, cc and I took a conservative approach.

The patch also changes Lexers.ocamldep_output to make it work with cpp's output (or ocamldep without -modules).

[whitequark]

Note: there is a bug in the patch:

I just found out that if there are two files "foo.c" and "foo.h", the .c file will never be picked up (internally, the extension is dropped).

There might be a very simple fix: try to copy both, if "foo.*" is being used, chances are both "foo.h" and "foo.c" are used. Unfortunately, but unfortunately I don't have the time to do it right now since I'm on the leave (for a place without a real internet access).