Code review for avoidance of direct cross-module links.

Code review for avoidance of direct cross-module links.

Remove $(pkglibdir) from $(rpathdir), since commits
d51924be886c2a05e691fa05b16cb6b30ab8370f and
eda04886c1e048d695728206504ab4198462168e 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.