gmain: Define fallback values for siginfo_t constants for musl

musl doesn’t define them itself, presumably because they’re not defined
in POSIX. glibc does define them. Thankfully, the values used in glibc
match the values used internally in other musl macros.

Define the values as a fallback. As a result of this, we can get rid of
the `g_assert_if_reached()` checks in `siginfo_t_to_wait_status()`.

This should fix catching signals from a subprocess when built against
musl.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2852

glib/gmain.c

@@ -79,6 +79,16 @@
 #ifndef W_STOPCODE
 #define W_STOPCODE(sig)      ((sig) << 8 | 0x7f)
 #endif
+#ifndef WCOREFLAG
+/* musl doesn’t define WCOREFLAG while glibc does. Unfortunately, there’s no way
+ * to detect we’re building against musl, so just define it and hope.
+ * See https://git.musl-libc.org/cgit/musl/tree/include/sys/wait.h#n51 */
+#define WCOREFLAG 0x80
+#endif
+#ifndef __W_CONTINUED
+/* Same as above, for musl */
+#define __W_CONTINUED 0xffff
+#endif
 #endif  /* HAVE_PIDFD */
 
 #ifdef G_OS_WIN32
@@ -5563,17 +5573,9 @@ siginfo_t_to_wait_status (const siginfo_t *info)
     case CLD_KILLED:
       return W_EXITCODE (0, info->si_status);
     case CLD_DUMPED:
-#ifdef WCOREFLAG
       return W_EXITCODE (0, info->si_status | WCOREFLAG);
-#else
-      g_assert_not_reached ();
-#endif
     case CLD_CONTINUED:
-#ifdef __W_CONTINUED
       return __W_CONTINUED;
-#else
-      g_assert_not_reached ();
-#endif
     case CLD_STOPPED:
     case CLD_TRAPPED:
     default: