Not only for typical cross-compilation build environments:
- allow to use CC definition from build environment
- ditto for PREFIX, packaging typically wants to use /usr
- use CPPFLAGS for include paths and just append
- let build environment decide about things like -Ox,
append rest of flags to CFLAGS
- use LDFLAGS, typically set by build environment
Signed-off-by: Joachim Foerster <joachim.foerster@missinglinkelectronics.com>