basic/missing_*.h: add asserts that the values are as expected

It's great that we provide fallback values, but if we got one of those wrong,
it could be a long time before anyone noticed. So let's add asserts that the
our internal defines actually match the official ones, when the latter are
available.

I did not add '#include "macro.h"' to missing_{audit,capability}, because
those are processed by an awk script that would need additional include
directories and could be confused by the additional lines. We don't include
those headers standalone anyway, so this is not necessary anyway.

src/basic/missing_timerfd.h

@@ -3,6 +3,10 @@
 
 #include <sys/timerfd.h>
 
+#include "macro.h"
+
 #ifndef TFD_TIMER_CANCEL_ON_SET
-#define TFD_TIMER_CANCEL_ON_SET (1 << 1)
+#  define TFD_TIMER_CANCEL_ON_SET (1 << 1)
+#else
+assert_cc(TFD_TIMER_CANCEL_ON_SET == (1 << 1));
 #endif