Remove $(pkglibdir) from $(rpathdir), since commits
d51924be88 and
eda04886c1 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.