[gtkmm] NMake Makefiles: Silence warnings C4251 and C4275

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

commit edafd4d840cfed2f60967a33cca24e13936699d1
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Fri Apr 10 14:52:23 2020 +0800

    NMake Makefiles: Silence warnings C4251 and C4275
    
    Since we are building against glibmm and pangomm (and soon atkmm) that
    have been converted to use __declspec(dllimport) for symbol import, we
    don't really need to bother about those warnings (and we are pretty much
    going to use the same CRT DLLs for all the -mm dependencies).

 MSVC_NMake/config-msvc.mak | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)
---
diff --git a/MSVC_NMake/config-msvc.mak b/MSVC_NMake/config-msvc.mak
index 0abf86c3..7e7c40aa 100644
--- a/MSVC_NMake/config-msvc.mak
+++ b/MSVC_NMake/config-msvc.mak
@@ -159,7 +159,7 @@ GTKMM_DEMO_DEP_LIBS = $(GTKMM_DEP_LIBS) $(EPOXY_LIB)
 GDKMM_BASE_CFLAGS =            \
        /Ivs$(VSVER)\$(CFG)\$(PLAT)     \
        /I..\gdk /I.\gdkmm      \
-       /wd4530 /std:c++17      \
+       /wd4251 /wd4275 /wd4530 /std:c++17      \
        /FImsvc_recommended_pragmas.h
 
 GTKMM_BASE_CFLAGS =            \
@@ -179,7 +179,6 @@ LIBGTKMM_CFLAGS =   \
        $(GTKMM_INCLUDES)
 
 GTKMM_DEMO_CFLAGS =    \
-       /wd4275 \
        $(GTKMM_BASE_CFLAGS)    \
        $(GTKMM_INCLUDES)