Change build of regress.so to use Makefile.shlib instead of depending

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.