do not change SECCOMP_RET defines if they're known

Signed-off-by: Kees Cook <keescook@chromium.org>
Makefile: clean up targets add "clean"

Signed-off-by: Kees Cook <keescook@chromium.org>
avoid gcc warning about type changes

Signed-off-by: Kees Cook <keescook@chromium.org>