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_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1b013f78fbdea47f1029482cedb5b390e672db14

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