patch 8.2.5087: cannot build with clang on MS-Windows

Problem:    Cannot build with clang on MS-Windows.
Solution:   Add support for building with clang. (Yegappan Lakshmanan,
            closes #10557)
diff --git a/src/GvimExt/Make_ming.mak b/src/GvimExt/Make_ming.mak
index 8d17142..5cde184 100644
--- a/src/GvimExt/Make_ming.mak
+++ b/src/GvimExt/Make_ming.mak
@@ -53,7 +53,9 @@
 WINDRES_FLAGS =
 LIBS :=  -luuid -lgdi32
 RES  := gvimext.res
+ifeq ($(findstring clang++,$(CXX)),)
 DEFFILE = gvimext_ming.def
+endif
 OBJ  := gvimext.o
 
 DLL  := gvimext.dll