Ok, so basename(3) is even more brain-damaged, inconsistent
and/or broken than dirname(3) on most systems, but there are
broken implementations of it out there.  Just use our already
existing internal parent_path() function instead and get rid
of the only place where we look for libgen.h.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7129 09075e82-0dd4-0310-85a5-a0d7c8717e4f