Log Message:
Change PKGNAME to libmpg123, and set the version to that of the
library's soname.  Split Makefile.common out from Makefile in preparation
for the addition of the mpg123 program and output module packages.
Change buildlink3.mk to reflect this, and rename options.mk
to mpg123-options.mk in adherence with convention, e.g.
gecko-options.mk.