Add HOMEPATH and DEFAULT_LIBDIR support

HOMEPATH allows setting the default for fs_homepath at build time.

DEFAULT_LIBDIR sets the default for the new fs_libpath, which can
be used to find libraries outside of the game path. This is necessary to
support UNIX style library placement.
Fix support for building tools without gcc

TOOLS_CC is set to gcc by default, update it to $(CC).

This was discovered as a result of the removal of gcc from FreeBSD
in favour of clang.