extmk.rb: include UPDATE_LIBRARIES

* ext/extmk.rb: include UPDATE_LIBRARIES in SUBMAKEOPTS for NMAKE
  which does not include macro definitions given by command line
  in MAKEFLAGS.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48204 b2dd03c8-39d4-4d8f-98ff-823fe69b080e