Fix CXX flags in various CMake build types

The flags we were defining for _DEBUG and _RELEASE were similar to the default
CMake flags for those types, which get appended to CMAKE_CXX_FLAGS, so we only
need to customize the latter.

The final flags will be for e.g. RelWithDebInfo:
${CMAKE_CXX_FLAGS} ${CXXFLAGS} ${CMAKE_CXX_FLAGS_RELWITHDEBINFO}
with ${CXXFLAGS} taken from the environment.