CFLAGS are reserved for external interaction via make variable, the
following should work:
$ make CFLAGS="-O3"
$ CFLAGS="-O3" make
1. Move internal flags to LOCAL_CFLAGS
2. Respect external CFLAGS
3. CFLAGS should be last compiler flags.
4. Default CFLAGS is -O optimization, remove OFLAGS.
5. Add WARNING_CFLAGS to control warning setting and enable to remove
if compiler does not support flags.
Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>