pgsql: Fix PGXS support for building loadable modules on AIX.

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.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6975dcddb23219bf842da7f622ea38d42e5adebf

Modified Files
--------------
src/backend/Makefile       |    3 +++
src/makefiles/Makefile.aix |    5 +++--
2 files changed, 6 insertions(+), 2 deletions(-)