Use the generic sigemptyset() available on both, glibc and musl libc
The SA_INTERRUPT is not defined in musl libc. Copy the (unused?)
definition from glibc's header bits/sigaction.h

--- src/prog/gpm-root.y	2012-10-26 23:21:38.000000000 +0200
+++ src/prog/gpm-root.y	2015-06-22 15:56:19.167982843 +0200
@@ -56,6 +56,9 @@
 #define minor(dev) ((dev)&0xff)
 #endif
 
+#if !defined(SA_INTERRUPT)
+# define SA_INTERRUPT 0x20000000 /* Historical no-op.  */
+#endif
 
 #define GPM_NULL_DEV "/dev/null"
 
@@ -1196,11 +1199,7 @@
                                                         LOG_DAEMON : LOG_USER);
    /* reap your zombies */
    childaction.sa_handler=reap_children;
-#if defined(__GLIBC__)
-   __sigemptyset(&childaction.sa_mask);
-#else /* __GLIBC__ */
-   childaction.sa_mask=0;
-#endif /* __GLIBC__ */
+   sigemptyset(&childaction.sa_mask);
    childaction.sa_flags=SA_INTERRUPT; /* need to break the select() call */
    sigaction(SIGCHLD,&childaction,NULL);