pgsql: Add an ifneq to avoid make warning on AIX --- there is a separate

Log Message:
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Add an ifneq to avoid make warning on AIX --- there is a separate rule
for postgres: on line 86, and line 43 shouldn't be used. Noted while
looking at kookaburra buildfarm results.

Modified Files:
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pgsql/src/backend:
Makefile (r1.109 -> r1.110)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/Makefile.diff?r1=1.109&r2=1.110)