Enable C compiler warnings
Enable C compiler optimizations

This results in a >2x performance boost in CRC32 calculations, see
https://gist.github.com/NicolasT/07eb1d584cfd66371f04 (note this patch doesn't
enable CPU-model-specific optimizations unlike the linked benchmarks, but the
difference is small).
Merge pull request #412 from Incubaid/c-compiler-options

C compiler options