Fix GH-1263: Name of GD DLL on Windows has changed in 8.0

Author: cmb69

reference/image/configure.xml

@@ -17,8 +17,9 @@
   the bundled one).
  </para>
  <para>
-  In Windows, you'll include the GD2 DLL <filename>php_gd2.dll</filename> as
+  In Windows, you'll include the GD DLL <filename>php_gd.dll</filename> as
   an extension in &php.ini;.
+  Prior to PHP 8.0.0, the DLL was named <filename>php_gd2.dll</filename>.
  </para>
 
  <para>