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_prctl.h

@@ -3,6 +3,8 @@
 
 #include <linux/prctl.h>
 
+#include "macro.h"
+
 /* 58319057b7847667f0c9585b9de0e8932b0fdb08 (4.3) */
 #ifndef PR_CAP_AMBIENT
 #define PR_CAP_AMBIENT 47
@@ -15,12 +17,19 @@
 
 /* b507808ebce23561d4ff8c2aa1fb949fe402bc61 (6.3) */
 #ifndef PR_SET_MDWE
-#define PR_SET_MDWE 65
+#  define PR_SET_MDWE 65
+#else
+assert_cc(PR_SET_MDWE == 65);
 #endif
+
 #ifndef PR_MDWE_REFUSE_EXEC_GAIN
-#define PR_MDWE_REFUSE_EXEC_GAIN 1
+#  define PR_MDWE_REFUSE_EXEC_GAIN 1
+#else
+assert_cc(PR_MDWE_REFUSE_EXEC_GAIN == 1);
 #endif
 
 #ifndef PR_SET_MEMORY_MERGE
-#define PR_SET_MEMORY_MERGE 67
+#  define PR_SET_MEMORY_MERGE 67
+#else
+assert_cc(PR_SET_MEMORY_MERGE == 67);
 #endif