mkmf.rb: --cflags to $CXXFLAGS

* 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/trunk@47457 b2dd03c8-39d4-4d8f-98ff-823fe69b080e