broadway backend: allow building on OpenBSD

On OpenBSD shm_open(3) is part of libc and there is not librt, so add an
according check.
Check if we have posix_fallocate before using it.

https://bugzilla.gnome.org/show_bug.cgi?id=728272

gdk/broadway/gdkbroadway-server.c

@@ -540,12 +540,14 @@ map_named_shm (char *name, gsize size)
   res = ftruncate (fd, size);
   g_assert (res != -1);
 
+#ifdef HAVE_POSIX_FALLOCATE
   res = posix_fallocate (fd, 0, size);
   if (res != 0)
     {
       shm_unlink (name);
       g_error ("Not enough shared memory for window surface");
     }
+#endif
   
   ptr = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);