patch 9.1.1275: MS-Windows: Not possible to pass additional flags to Make_mvc

Problem:  MS-Windows: Not possible to pass additional flags to Make_mvc
Solution: Introduce $CI_FLAGS and use it to pass additional flags for
          the Github CI in order to treat size conversion warnings
          (C4267) as errors (Yegappan Lakshmanan)

closes: #17028

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 2825865..ab6bd6c 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -533,7 +533,7 @@
 CFLAGS = -c /W3 /GF /nologo -I. -Iproto -DHAVE_PATHDEF -DWIN32 -DHAVE_STDINT_H \
 		$(CSCOPE_DEFS) $(TERM_DEFS) $(SOUND_DEFS) $(NETBEANS_DEFS) \
 		$(NBDEBUG_DEFS) $(XPM_DEFS) $(SOD_DEFS) $(SOD_INC) \
-		$(CHANNEL_DEFS) $(DEFINES) \
+		$(CHANNEL_DEFS) $(DEFINES) $(CI_CFLAGS) \
 		-DWINVER=$(WINVER) -D_WIN32_WINNT=$(WINVER) \
 		/source-charset:utf-8