[gtkmm] NMake Makefiles: Use /utf-8

[Chun-wei Fan <fanchunwei src gnome org>]

commit f95255ea1a898da1ce215f1f9862d3c6faba4019
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Thu Jun 4 17:48:29 2020 +0800

    NMake Makefiles: Use /utf-8
    
    This makes it easier to build gtkmm on East Asian locales so that we can avoid
    unicode handling issues in the compiler, which can lead to errors and/or broken
    builds, without needing to re-set the system non-Unicode locale settings to a
    Western/English locale.

 MSVC_NMake/config-msvc.mak | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/MSVC_NMake/config-msvc.mak b/MSVC_NMake/config-msvc.mak
index 7e7c40aa..f9f9f447 100644
--- a/MSVC_NMake/config-msvc.mak
+++ b/MSVC_NMake/config-msvc.mak
@@ -160,7 +160,7 @@ GDKMM_BASE_CFLAGS =         \
        /Ivs$(VSVER)\$(CFG)\$(PLAT)     \
        /I..\gdk /I.\gdkmm      \
        /wd4251 /wd4275 /wd4530 /std:c++17      \
-       /FImsvc_recommended_pragmas.h
+       /FImsvc_recommended_pragmas.h /utf-8
 
 GTKMM_BASE_CFLAGS =            \
        /Ivs$(VSVER)\$(CFG)\$(PLAT)     \