disable gcc's __attribute__((warn_unused_result))

Casting to (void) to discard a function return value doesn't satisfy
gcc's -Wunused-result (which we aren't enabling but is apparently
being activated for particular functions by glibc header files).  Turn
it into a no-op to suppress three dozen warnings from Travis builds.

include/tradstdc.h

@@ -405,6 +405,10 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #if __GNUC__ >= 3
 #define UNUSED __attribute__((unused))
 #define NORETURN __attribute__((noreturn))
+/* disable gcc's __attribute__((__warn_unused_result__)) since explicitly
+   discarding the result by casting to (void) is not accepted as a 'use' */
+#define __warn_unused_result__ /*empty*/
+#define warn_unused_result /*empty*/
 #endif
 #endif