musl doesn't have an internal __sighandler_t type,
and only makes sighandler_t visible if _GNU_SOURCE
is defined.
https://bugs.gentoo.org/714026

--- a/src/sunlink.c
+++ b/src/sunlink.c
@@ -102,9 +102,6 @@
 static volatile int SIGINT_received = 0;
 #if defined(__unix__)
 #include <signal.h>
-#if defined(__linux__) && !defined(__USE_GNU)
-typedef __sighandler_t sighandler_t;
-#endif
 #if defined(__FreeBSD__) || defined(__OpenBSD__)
 typedef sig_t sighandler_t;
 #endif