[atkmm] NMake Makefiles: Use /utf-8

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

commit 5e8213d4bbc6b316e46a21c463c476c16bd8bb09
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Fri Feb 7 19:02:22 2020 +0800

    NMake Makefiles: Use /utf-8
    
    This avoids error C4819 due to unicode handling issues in the compiler,
    which is often raised when attempting to build in East-Asian locales
    (Chinese, Japanese and Korean).  Since Visual Studio 2017 is required to
    build the C++17/master branch of atkmm, this is enabled for all builds.

 MSVC_NMake/config-msvc.mak | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/MSVC_NMake/config-msvc.mak b/MSVC_NMake/config-msvc.mak
index 38e2638..b2df058 100644
--- a/MSVC_NMake/config-msvc.mak
+++ b/MSVC_NMake/config-msvc.mak
@@ -25,6 +25,7 @@ DEBUG_SUFFIX =
 ATKMM_BASE_CFLAGS =            \
        /I..\atk /I.\atkmm      \
        /wd4530 /std:c++17      \
+       /utf-8  \
        /FImsvc_recommended_pragmas.h
 
 ATKMM_EXTRA_INCLUDES = \