Inpompatible function pointers are an error instead of a warning by default since clang16

Bug: https://bugs.gentoo.org/896232

Pascal Jäger <pascal.jaeger@leimstift.de> (2023-09-22)

--- a/window.c
+++ b/window.c
@@ -833,7 +833,7 @@ char imageInWindow (Display *disp, int scrn, Image *image, OptionSet *global_opt
       /* reset alarm to -delay seconds after every event */
 #ifdef ENABLE_TIMEOUT
       AlarmWentOff = 0;
-      signal(SIGALRM, delayAlarmHandler);
+      signal(SIGALRM, (__sighandler_t)delayAlarmHandler);
       alarm(delay);
 #endif /* ENABLE_TIMEOUT */
   }