Strict C warnings

[MisterDA]

Enable warnings when compiling C code

In particular, enable warnings for stricter C.
See https://lwn.net/Articles/913505/.

Fix strict-prototypes warnings

In all cases, the functions don't expect argument, so replacing with
void is fine. Otherwise, the functions would accept any number of
arguments.

[aantron]

Thank you! I've merged the void function signatures. I haven't merged the warnings flags, because it seems to me like that would be a potential portability issue at the compiler command line interface. I tested what happens when gcc receives a warning flag that it does not recognize, and it fails. It seems that in the general case, we would have to have a discovery script to find out which warnings flags our compiler supports, but I'd like to avoid that. Please let me know if any of this is inaccurate. I'll propagate te fixed prototypes to shims.h.

[MisterDA]

You're right, this was an oversight from me. It's ok as-is with the C patches.