make: LOCAL_CXXFLAGS is obsoleted, replaced with LOCAL_CPPFLAGS.

LOCAL_CXXFLAGS is just an alias for LOCAL_CPPFLAGS, but if you read the
docs in NDK relating to Android.mk, it states that this alias is
obsolete and that there's no guarantee it will continue to be in the
NDK. It's already compatible with C++98 and C++03.