#elsif

[Paul Eggert]

   Date: Tue, 7 Apr 1998 12:08:20 +0200 (MET DST)
   From: "Philippe De Muyter" <phdm@macqel.be>

   #if defined(A)
	   printf("A defined\n");
   #elsif  defined(B)
	   printf("B undefined\n");
   #else   /* nor A nor B */
	   printf("nor A nor B defined\n");
   #endif

As I read the C standard, if A is not defined, then GCC is required to
accept that program fragment, though I admit the standard's wording is
not entirely clear.

It might be useful for cpp to warn about skipped, unknown directives
if -W or -Wall is specified.