gmake.mk: mflags without -jN

* defs/gmake.mk (mflags): override the definition in common.mk
  without -jN option.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57635 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

defs/gmake.mk

@@ -1,7 +1,7 @@
 # -*- makefile-gmake -*-
 gnumake = yes
 override gnumake_recursive := $(if $(findstring n,$(firstword $(MFLAGS))),,+)
-mflags := $(filter-out -j%,$(MFLAGS))
+override mflags := $(filter-out -j%,$(MFLAGS))
 
 CHECK_TARGETS := exam love check%
 # expand test targets, and those dependents