merge revision(s) r47457: [Backport #8315]

	* lib/mkmf.rb (MakeMakefile#pkg_config): append --cflags to also
	  $CXXFLAGS, as they are often used by C++ compiler.
	  [ruby-core:54532] [Bug #8315]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@47587 b2dd03c8-39d4-4d8f-98ff-823fe69b080e