merge revision(s) r44459,r44470: [Backport #8315]

	* lib/mkmf.rb (configuration): Make CXXFLAGS customizable.
	  Patch by Kohei Suzuki (eagletmt). [Fixes GH-492]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@47592 b2dd03c8-39d4-4d8f-98ff-823fe69b080e