Fix support for libunity in configure

This should fix http://sourceforge.net/p/scummvm/bugs/6688/
Two things:
1. For some reason we asked for unity exactly == 3.8.4.
Bad for everyone who isn't stuck in 2011.
2. If unity was explicity enabled BUT we weren't explictly provided a prefix, we didn't set LIBUNITY_LIBS and -_CFLAGS

There might be other problems, of course, as @wjp noted