mkmf.rb: no CFLAGS in CXXFLAGS

* lib/mkmf.rb (configuration): not include all CFLAGS in CXXFLAGS, to
  use different set than C for C++.  [ruby-core:45273] [Bug #6504]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39994 b2dd03c8-39d4-4d8f-98ff-823fe69b080e