Remove source paths from build

[jprjr]

This helps with reproducible builds by making __FILE__ resolve to just the source filename instead of the full path (which can differ from build to build).

[daurnimator]

While this does seem to help, the build artifact still has my build directory in it:

$ strings before.so | grep daurn
/home/daurnimator/src/luaossl/src/openssl.c
/home/daurnimator/src/luaossl/src
/home/daurnimator/src/luaossl/src/openssl.c
/home/daurnimator/src/luaossl

$ strings after.so | grep daurn
/home/daurnimator/src/luaossl