pgsql: Change build of regress.so to use Makefile.shlib instead of

Log Message:
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Change build of regress.so to use Makefile.shlib instead of depending
on the not-very-good .so pattern rules in the port-specific Makefiles.
(This leaves only pgxs' MODULES case needing those rules.)  Also,
compile pgsleep.c locally and add it to regress.so to avoid failure
on AIX.

Modified Files:
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    pgsql/src/test/regress:
        GNUmakefile (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/GNUmakefile.diff?r1=1.50&r2=1.51)