Is using the name "CPP" for preprocessor a good idea? Last time I checked, CPP used to mean "C Plus Plus" (confused)
The other thing is they're somewhat standard for unix Makefiles. If there are environment variables defined
CFLAGS - for C flags
LDFLAGS - for LoaDer (linker) flags
CPPFLAGS - for C PreProcessor flags
then a lot of makefile will use those variables to pass to gcc et al. Guess the short cryptic names dates back to early unix lazyness.