* common.mk (mflags): pass make flags to sub-makes, for nmake
  which cannot pass them by the environment variable.

* defs/gmake.mk (mflags): filter out -j option for sub-makes.

* template/exts.mk.tmpl (MFLAGS): extract MFLAGS from sub extmk
  files for nmake.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e