[glibmm] NMake Makefiles: Use /utf-8 where available

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

commit 15540dc1dd58ee9ab03068ef5d7222d0431dfffe
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Thu Dec 26 18:19:14 2019 +0800

    NMake Makefiles: Use /utf-8 where available
    
    This avoids miscompilations and errors when building under certain
    locales (Chinese, Korean and Japanese in particular) as the compiler
    could not handle Unicode sequences properly without this flag.

 MSVC_NMake/detectenv-msvc.mak | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/MSVC_NMake/detectenv-msvc.mak b/MSVC_NMake/detectenv-msvc.mak
index 9a92c110..e7ae937d 100644
--- a/MSVC_NMake/detectenv-msvc.mak
+++ b/MSVC_NMake/detectenv-msvc.mak
@@ -123,6 +123,10 @@ CFLAGS_ADD = $(CFLAGS_ADD_NO_GL) /GL
 CFLAGS_ADD = $(CFLAGS_ADD) /d2Zi+
 CFLAGS_ADD_NO_GL = $(CFLAGS_ADD_NO_GL) /d2Zi+
 !endif
+!if $(VSVER) >= 14
+CFLAGS_ADD = $(CFLAGS_ADD) /utf-8
+CFLAGS_ADD_NO_GL = $(CFLAGS_ADD_NO_GL) /utf-8
+!endif
 !else
 CFLAGS_ADD = /MDd /Od
 CFLAGS_ADD_NO_GL = $(CFLAGS_ADD)