Some libc's may implement tgammaf as a header macro using tgamma(), so don't assume it'll be in the library.
For the sake of older versions of gcc (and other compilers), don't use the #warning CPP directive, nor the -Wno-error=cpp option. Also, fix a strict alias warning in modffi.c for older compilers, and add a test for ffi module. Addresses issue #847.