patch 8.0.0881: win32.mak no longer included in Windows SDK

Problem:    win32.mak no longer included in Windows SDK.
Solution:   Do not include win32.mak. (Ken Takata)
diff --git a/src/GvimExt/Makefile b/src/GvimExt/Makefile
index 043d37d..c681d33 100644
--- a/src/GvimExt/Makefile
+++ b/src/GvimExt/Makefile
@@ -36,8 +36,16 @@
 
 !ifdef SDK_INCLUDE_DIR
 !include $(SDK_INCLUDE_DIR)\Win32.mak
-!else
+!elseif "$(USE_WIN32MAK)"=="yes"
 !include <Win32.mak>
+!else
+cc = cl
+link = link
+rc = rc
+cflags = -nologo -c
+lflags = -incremental:no -nologo
+rcflags = /r
+olelibsdll = ole32.lib uuid.lib oleaut32.lib user32.lib gdi32.lib advapi32.lib
 !endif
 
 # include CPUARG