Fix runtime errors on 64-bit architectures (tested under MacOS Catalina).

* Ignoring implicit function declaration warnings resulted in pointers
   being clobbered to int type.

libiberty/vfork.c

@@ -1,6 +1,8 @@
 /* Emulate vfork using just plain fork, for systems without a real vfork.
    This function is in the public domain. */
 
+int fork ();
+
 int
 vfork ()
 {