Link C/C++ stlib statically for binary gems
    
    Also:
    1. Compile via `mkmf` instead of libsass's own Makefile.
       This is necessary to make cross-compilation work (rake-compiler hooks
       into mkmf).
    2. Do not use per-ruby versions of `libsass.so`. It is unnecessary,
       because `libsass.so` is Ruby-agnostic (the FFI gem is used instead to
       use it from any Ruby version).
    3. Add VERSION file to the non-precompiled gem.
    4. Clean before every build.