Define Assert() et al to ((void)0) to avoid pedantic warnings.

gcc's -Wempty-body warns about the current usage when compiling
postgres without --enable-cassert.

src/include/c.h

@@ -578,12 +578,12 @@ typedef NameData *Name;
  */
 #ifndef USE_ASSERT_CHECKING
 
-#define Assert(condition)
+#define Assert(condition)	((void)true)
 #define AssertMacro(condition)	((void)true)
-#define AssertArg(condition)
-#define AssertState(condition)
+#define AssertArg(condition)	((void)true)
+#define AssertState(condition)	((void)true)
 #define AssertPointerAlignment(ptr, bndr)	((void)true)
-#define Trap(condition, errorType)
+#define Trap(condition, errorType)	((void)true)
 #define TrapMacro(condition, errorType) (true)
 
 #elif defined(FRONTEND)