common.mk: fix win32/ifchange.bat usage
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55340 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
common.mk: fix win32/ifchange.bat usage
also needs quotes to escape an equal sign.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55340 b2dd03c8-39d4-4d8f-98ff-823fe69b080e