CFLAGS_FOR_TARGET and CXXFLAGS_FOR_TARGET are rewritten in gcc-4.8.x and
gcc-4.9.x, so libstdc++ does not get any flags passed to gcc configure.
Backport fixes for config/mt-gnu and config/mt-ospace that preserve these
flags. With these fixes libstdc++ gets built with flags specified in
CT_TARGET_CFLAGS.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
