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/src/Make_mvc.mak b/src/Make_mvc.mak
index 65da331..d5159ab 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -26,7 +26,7 @@
 #	GUI interface: GUI=yes (default is no)
 #
 #	GUI with DirectWrite (DirectX): DIRECTX=yes
-#	  (default is no, requires GUI=yes and MBYTE=yes)
+#	  (default is yes if GUI=yes, requires GUI=yes and MBYTE=yes)
 #
 #	Color emoji support: COLOR_EMOJI=yes
 #	  (default is yes if DIRECTX=yes, requires WinSDK 8.1 or later.)
@@ -180,6 +180,10 @@
 
 TARGETOS = WINNT
 
+!ifndef DIRECTX
+DIRECTX = $(GUI)
+!endif
+
 # Select one of eight object code directories, depends on GUI, OLE, DEBUG and
 # interfaces.
 # If you change something else, do "make clean" first!