merge revision(s) 42223: [Backport #8702]

	* lib/mkmf.rb (xsystem): expand environment variable in all macros not
	  expanded with RbConfig.  [Bug #8702]

	* test/mkmf/test_framework.rb (create_framework): replace all $@ not
	  only once.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@42358 b2dd03c8-39d4-4d8f-98ff-823fe69b080e