Rename CFLAGS and friends to follow the conventions of the implicit rules
defined by GNU Make:
- $(CPPFLAGS) is passed to both C++ and C compiler
- $(CXXFLAGS) is passed to C++ compiler only
- $(CFLAGS) is passed to C compiler only
- C++ compiler is in $(CXX)
Signed-off-by: Jan Lindemann <jan@janware.com>