patch 8.0.1608: Win32: directx not enabled by default

Problem:    Win32: directx not enabled by default.
Solution:   Change Makefile to enable directx by default. (Ken Takata)
diff --git a/runtime/doc/various.txt b/runtime/doc/various.txt
index 0e1e1ab..4b1c853 100644
--- a/runtime/doc/various.txt
+++ b/runtime/doc/various.txt
@@ -352,7 +352,7 @@
 N  *+dialog_con_gui*	Support for |:confirm| with GUI and console dialog.
 N  *+diff*		|vimdiff| and 'diff'
 N  *+digraphs*		|digraphs| *E196*
-m  *+directx*		Win32 GUI only: DirectX and |'renderoptions'|
+   *+directx*		Win32 GUI only: DirectX and |'renderoptions'|
    *+dnd*		Support for DnD into the "~ register |quote_~|.
 B  *+emacs_tags*	|emacs-tags| files
 N  *+eval*		expression evaluation |eval.txt|