Prevent passing down MAKELEVEL/MAKEFLAGS from non-GNU make to GNU make.

FreeBSD's make, for one, sets the MAKELEVEL environment variable when
invoking commands.  In the special Makefile we provide to hand off control
from a non-GNU make to GNU make, this causes GNU make to think it is a
child make invocation rather than top-level.  That interferes with the hack
added in commit dcae5fac to cause the temp-install tree to be made only by
the top-level invocation of gmake.  Unset the variable to prevent that.

Likewise unset MAKEFLAGS, which FreeBSD's make also sets, and which could
easily confuse gmake.  There are no reports of actual trouble from that,
but it seems better to be proactive.

Back-patch to 9.5 where dcae5fac came in.

Thomas Munro, hacked a bit more by me

Discussion: https://postgr.es/m/CAEepm=1ueww35AXTkt1A3gyzZUqv5XCzh8RUNvJZAQAW=eOhVw@mail.gmail.com

Makefile

@@ -29,6 +29,7 @@ all check install installdirs installcheck installcheck-parallel uninstall clean
 	\
 	 if [ x"$${GMAKE+set}" = xset ]; then \
 	   echo "Using GNU make found at $${GMAKE}"; \
+	   unset MAKEFLAGS; unset MAKELEVEL; \
 	   $${GMAKE} $@ ; \
 	 else \
 	   echo "You must use GNU make to build PostgreSQL." ; \