[gtkmm] MSVC_NMake/README: Update build info

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

commit bcb99ee9acbb89fcfbf7cb3149e10e6391dfc13f
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Tue Jun 30 15:11:06 2020 +0800

    MSVC_NMake/README: Update build info
    
    Note that it is possible to build with Visual Studio with Meson, on the
    condition that the -mm dependencies are built with Meson as well, since we need
    their pkg-config files.
    
    Also note that the same compiler version should be used to build gtkmm and the
    -mm dependencies, even for Visual Studio 2017 and 2019.

 MSVC_NMake/README | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)
---
diff --git a/MSVC_NMake/README b/MSVC_NMake/README
index 86b3dc63..88fb8cda 100644
--- a/MSVC_NMake/README
+++ b/MSVC_NMake/README
@@ -1,8 +1,13 @@
-Building gtkmm-3.0 with Visual Studio 2017
+Building gtkmm-4.0 with Visual Studio 2017
 
-* You will need Visual Studio 2017 (MSVC 15.0). Earlier versions of the compiler, including 14.0 and earlier 
will not work.
-* Install the latest Win32 GTK+ Development files from ftp://ftp.gnome.org/pub/GNOME/binaries/win32/gtk+
-* Build libsigc++, glibmm, cairomm, atkmm and pangomm from source.
+* You will need Visual Studio 2017 (MSVC 15.0). Earlier versions of the compiler, including 14.0 and
+  earlier will not work.  The build can be carried out using the NMake Makefiles supplied in this
+  directory, or via using Meson.  Meson is only supported if libsigc++, glibmm, cairomm, atkmm and pangomm
+  are built with Meson as well.
+* Install the latest Win32 GTK+ 4.x Development files.
+* Build libsigc++, glibmm, cairomm, atkmm and pangomm from source, with the same compiler version, even if
+  on Visual Studio 2017 or 2019.  If there are strange linker errors, they are likely caused by mixing
+  different compiler versions.
 * Use the Visual Studio command prompt that is appropriate for your build and navigate to this directory.
 * Run nmake /f Makefile.vc CFG=[debug|release].  Also pass in PREFIX=<path> to this command line if
   GTK+, libsigc++, glibmm, cairomm, atkmm and pangomm are not in $(srcroot)\..\vs15\$(platform).  You
@@ -24,4 +29,4 @@ Armin Burgmeier
 10/02/2010
 
 Chun-wei Fan
-09/10/2015
+09/10/2015