mkmf.rb: --cflags to $CXXFLAGS
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47457 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: --cflags to $CXXFLAGS
$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