Silence unused result warnings on newer compilers

Newer versions of GCC and Clang are not satisfied by a cast to void,
this fix is adapted from glibc's solution.

New wrapper function ignore_result should be used when a function with
explicit _unused_attribute_ wrapper is called whose result will not be
handled.

src/wutil.cpp

@@ -338,7 +338,7 @@ void safe_perror(const char *message) {
     safe_append(buff, safe_strerror(err), sizeof buff);
     safe_append(buff, "\n", sizeof buff);
 
-    (void)write(STDERR_FILENO, buff, strlen(buff));
+    ignore_result(write(STDERR_FILENO, buff, strlen(buff)));
     errno = err;
 }