work around ubuntu 20.10 build issue

NetHack was trying to suppress warn_unused_result
in include/tradstdc.h, by defining warn_unused_result
to an empty string. That began causing a build error
in a system-supplied header file cdefs.h
when using 20.10 ubuntu impish.

Try skipping that in tradstdc.h for any linux, unless
the NetHack build defines GCC_URWARN to force it into
play.

include/tradstdc.h

@@ -405,12 +405,14 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #if __GNUC__ >= 3
 #define UNUSED __attribute__((unused))
 #define NORETURN __attribute__((noreturn))
+#if !defined(__linux__) || defined(GCC_URWARN)
 /* 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
+#endif
 
 #ifndef PRINTF_F
 #define PRINTF_F(f, v)