So user could specify outside CFLAGS values.

Cc: Thomas Renninger <trenn@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>