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_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3aa082f37aca0b14ffdd90167e236c228d73b586

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