CPPFLAGS are not related to c++ but are preprocessor flags (like: -I/opt/something/include). c++ compiler options would be CXXFLAGS.
as for creating a patch, i would but if you wanna switch to cmake anyway it might not be necessary anymore. a word of caution though regarding cmake:
in many cases the problem with cmake is cmake itself because it has quite hefty compiler requirements. ironically often much more hefty than the actual program it's supposed to prepare. in case if citadel that would be such a case. so if compatibility is important to you, cmake is not a good choice.
