Improve linking with widget libraries on macOS

This allows getting rid of the vendor shell hack in libXaw and libXaw3d
by using a coalesced weak reference.

src/Vendor.c

@@ -82,6 +82,9 @@ DllMain(unsigned long mod_handle, unsigned long flag, void *routine)
 #endif
 
 /* *INDENT-OFF* */
+#if defined(__APPLE__)
+__attribute__((weak))
+#endif
 externaldef(vendorshellclassrec)
 VendorShellClassRec vendorShellClassRec = {
     {
@@ -137,5 +140,8 @@ VendorShellClassRec vendorShellClassRec = {
 };
 /* *INDENT-ON* */
 
+#if defined(__APPLE__)
+__attribute__((weak))
+#endif
 externaldef(vendorshellwidgetclass)
 WidgetClass vendorShellWidgetClass = (WidgetClass) (&vendorShellClassRec);