configure: also set the CMAKE_C*_FLAGS_DEBUG and CMAKE_C*_FLAGS_RELEASE of CXX_FLAGS is specified

for me, setting only CMAKE_CXX_FLAGS did not do the trick when I was
trying to compile everyting with libstdc++'s debug mode enabled. (it
caused linker errors because opm-parser was not linked against
std::debug in this case.) Maybe I got something wrong, though...