Add support for C-specific qualifiers and syntax not supported by C++

[staticintlucas]

I understand this project is mainly focused on C++, but I'm trying to move a C project away from Doxygen and figured I'd give Standardese a try.

My code makes use of the C restrict qualifier which Standardese seems to recognise as an identifier. This causes a ton of expected ')' errors. The restrict and _Atomic qualifiers are one of the few parts of C which have not made their way into any C++ standard.

The only other C-specific syntax that I can think of is static array indices, but currently the error comes from libclang saying static array size is a C99 feature, not permitted in C++. So hopefully if libclang is told to parse C code everything else will just work.

If you're happy to add support I can work on a PR. The additional qualifiers would require some upstream changes in foonathan/cppast, while static array indices would probably just require allowing C standards in --compilation.standard and passing that to libclang

[foonathan]

I welcome any PRs to cppast.