pgsql: When calling unsupported "make check" with a pgxs module, return

Log Message:
-----------
When calling unsupported "make check" with a pgxs module, return a nonzero
exit code.

Modified Files:
--------------
pgsql/src/makefiles:
pgxs.mk (r1.15 -> r1.16)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/makefiles/pgxs.mk?r1=1.15&r2=1.16)