We should test with gcc and compcert's ccomp

[mattam82]

Our opam file will require clang for now (see PR #62 ) , as gcc is usually too slow and we barely tested CompCert's ccomp AFAIK.
We should add a Coq option to choose which C compiler to use (for Eval/Run) and use the same one in the Makefiles.
Probably we need CI tests for all three as well.