(signal_delivery_thread): Restrict scm_i_pthread_sigmask

to HAVE_PTHREAD_SIGMASK, it doesn't exist on mingw.  Reported by Nils
Durner.

libguile/scmsigs.c

@@ -115,6 +115,12 @@ close_1 (SCM proc, SCM arg)
 }
 
 #if SCM_USE_PTHREAD_THREADS
+/* On mingw there's no notion of inter-process signals, only a raise()
+   within the process itself which apparently invokes the registered handler
+   immediately.  Not sure how well the following code will cope in this
+   case.  It builds but it may not offer quite the same scheme-level
+   semantics as on a proper system.  If you're relying on much in the way of
+   signal handling on mingw you probably lose anyway.  */
 
 static int signal_pipe[2];
 
@@ -158,12 +164,13 @@ read_without_guile (int fd, char *buf, size_t n)
 static SCM
 signal_delivery_thread (void *data)
 {
-  sigset_t all_sigs;
   int n, sig;
   char sigbyte;
-
+#if HAVE_PTHREAD_SIGMASK  /* not on mingw, see notes above */
+  sigset_t all_sigs;
   sigfillset (&all_sigs);
   scm_i_pthread_sigmask (SIG_SETMASK, &all_sigs, NULL);
+#endif
 
   while (1)
     {