__signalhandler_t isn't defined on musl, one needs to drill down to bare type definition
so code builds both on glibc and musl systems
https://bugs.gentoo.org/924114
--- 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, (void (*)(int))delayAlarmHandler);
       alarm(delay);
 #endif /* ENABLE_TIMEOUT */
   }