Looks like X_ShmAttach is considered private to the library and has been removed from its headers.
Just hardcode the value, it's only usd to print some help in case of an shm attach error.
--- x11vnc-0.9.8/x11vnc/cleanup.c
+++ x11vnc-0.9.8/x11vnc/cleanup.c
@@ -276,6 +276,10 @@
	return 0;
 }

+#ifndef X_ShmAttach
+#define X_ShmAttach 1
+#endif
+
 static int Xerror(Display *d, XErrorEvent *error) {
	X_UNLOCK;