For linkpath, add PREFIX/usr/lib:PREFIX/lib in that order,
but to runpath, add in reverse order.
This also is needed for AIX to allow (manual) workarounds, when updated
shared libraries have SONAME support while the old one does not, because
'libX.so' cannot be loaded any more when having SONAME support, as it
becomes either (a symlink to) an archive or a standalone import file.
This is related to http://bugs.gentoo.org/show_bug.cgi?id=213277