libtool fix for OS X

Make autogen.sh work on Darwin/OS X by checking for glibtool/glibtoolize first, then libtool/glibtool.

The libtool binary that OS X uses is actually not the same version that other unices use. The version that other unices uses is named glibtool on OS X. It seems that syn68k uses stuff that isn't available on OS X's/Darwin's version.