build-sys(win32): enable msvc preprocessor conformance

(cherry picked from commit ffae8537f704c6c6f9c7bd5bac2418146bf5269f)

Author: edam

mk_mvc.mak

@@ -13,7 +13,7 @@ include source.mak
 COMMON_DEFINES =
 DEFINES = -DWIN32 $(COMMON_DEFINES) -DHAVE_REPOINFO_H -DHAVE_PACKCC -DREADTAGS_DSL
 INCLUDES = -I. -Ignulib -Imain -Iparsers -Ilibreadtags -Idsl
-OPT = /O2 /WX
+OPT = /O2 /WX /Zc:preprocessor
 PACKCC = packcc.exe
 GNULIB_OBJS = $(MVC_GNULIB_SRCS:.c=.obj)
 WIN32_OBJS = $(WIN32_SRCS:.c=.obj)