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>