Do not #include external header files when compiling via C
This has several advantages: - -fvia-C is consistent with -fasm with respect to FFI declarations: both bind to the ABI, not the API. - foreign calls can now be inlined freely across module boundaries, since a header file is not required when compiling the call. - bootstrapping via C will be more reliable, because this difference in behavour between the two backends has been removed. There is one disadvantage: - we get no checking by the C compiler that the FFI declaration is correct. So now, the c-includes field in a .cabal file is always ignored by GHC, as are header files specified in an FFI declaration. This was previously the case only for -fasm compilations, now it is also the case for -fvia-C too.