Copied from linux kernel, 'make W=1' will build with various additional
warnings turned on. There are 3 levels, combinations are possible. The
build is quite noisy, not all warnings need to be fixed.
A specific warning can be turned on by 'make EXTRA_CFLAGS=-Wsomething'.
Signed-off-by: David Sterba <dsterba@suse.com>
David Sterba