Remove $(pkglibdir) from $(rpathdir), since commits
d51924be and
eda04886 removed direct linkage to
objects stored there.  Users are unlikely to notice the difference.
Accompany every $(python_libspec) with $(python_additional_libs); this
doesn't fix a demonstrated bug, but it might do so on rare Python
configurations.  With these changes, AIX ceases to be a special case.