Prevent implicit -Wsystem-headers passing from CFLAGS to CXXFLAGS.
Upcoming clang will be the first base system part that very heavily depends on
c++ headers, this prevents setting any level of WARNS in clang's makefiles
without performing certain unsafe manipulations of CXXFLAGS. To prevent that
it makes sense to allow only explict use directly in CXXFLAGS where it is
needed given c++11 headers are part of contrib and quite noisy with deprecated
attributes for previous features/interfaces to c++11 standard.