patch 8.0.0638: cannot build with new MSVC version

Problem:    Cannot build with new MSVC version VS2017.
Solution:   Change the compiler arguments. (Leonardo Manera, closes #1731,
            closes #1747)
diff --git a/src/GvimExt/Makefile b/src/GvimExt/Makefile
index 798efd2..043d37d 100644
--- a/src/GvimExt/Makefile
+++ b/src/GvimExt/Makefile
@@ -1,11 +1,14 @@
 # Makefile for GvimExt, using MSVC
 # Options:
 #   DEBUG=yes		Build debug version (for VC7 and maybe later)
+#   CPUARG=		/arch:IA32/AVX/etc, call from main makefile to set
+#   			automatically from CPUNR
 #
 
-TARGETOS=WINNT
+TARGETOS = WINNT
+
 !ifndef APPVER
-APPVER=5.0
+APPVER = 5.01
 !endif
 
 !if "$(DEBUG)" != "yes"
@@ -37,6 +40,9 @@
 !include <Win32.mak>
 !endif
 
+# include CPUARG
+cflags = $(cflags) $(CPUARG)
+
 all: gvimext.dll
 
 gvimext.dll:    gvimext.obj	\