Fix PGXS support for building loadable modules on AIX.
Building a shlib on AIX requires use of the mkldexport.sh script, but we failed to install that, preventing its use from non-source-tree contexts. Also, Makefile.aix had the wrong idea about where to find the installed copy of the postgres.imp symbol file used by AIX.
Per report from John Pierce. Patch all the way back, since this has been broken since the beginning of PGXS.