Don't unset MAKEFLAGS in non-GNU Makefile.

It's useful to be able to pass down options like -s and -j.

Back-patch to 9.5, like commit a76200de.

Discussion: https://postgr.es/m/CA%2BhUKG%2Be1M8-BbL%3DPqhTp6oO6XPO6%2Bs9WGQMLfbuZ%3DG9CtzyXg%40mail.gmail.com

Makefile

@@ -33,7 +33,7 @@ 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; \
+	   unset MAKELEVEL; \
 	   $${GMAKE} $@ ; \
 	 else \
 	   echo "You must use GNU make to build PostgreSQL." ; \