Don't unset MAKEFLAGS in non-GNU Makefile.
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

lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb