This is backported partially from 2.20 branch

http://sourceware.org/ml/binutils-cvs/2009-10/msg00173.html

-Khem
Index: binutils-2.20/gas/as.h
===================================================================
--- binutils-2.20.orig/gas/as.h	2009-10-22 00:27:08.700469349 -0700
+++ binutils-2.20/gas/as.h	2009-10-22 00:28:04.068416318 -0700
@@ -238,7 +238,7 @@ typedef addressT valueT;
 #define know(p) gas_assert(p)	/* Verify our assumptions!  */
 #endif /* not yet defined */
 #else
-#define know(p)			/* know() checks are no-op.ed  */
+#define know(p) do {} while (0)	/* know() checks are no-op.ed  */
 #endif
 
 /* input_scrub.c */