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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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/facc2047feda0c6fc4d2028f36f5a73d6c5cfc76

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