[glibmm/glibmm-2-64] Update README.win32

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

commit 50dc004c9523b402dc93642dbe7914ff1605b303
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Fri Jun 12 21:56:45 2020 +0800

    Update README.win32
    
    Let people know that it is recommended (and even required at some
    points) to use the same Visual Studio version for building glibmm and
    dependent software, even for Visual Studio 2015, 2017 and 2019.

 README.win32 | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)
---
diff --git a/README.win32 b/README.win32
index 62bcea3d..f3846532 100644
--- a/README.win32
+++ b/README.win32
@@ -45,7 +45,7 @@ make
 make check
 make install
 
-2. MS Visual Studio 2015
+2. MS Visual Studio 2015~2019
 
 In a Visual Studio command prompt, navigate to the MSVC_NMake directory.
 Run 'nmake /f Makefile.vc CFG=[release|debug]' to build the glibmm and
@@ -58,7 +58,18 @@ C++ dependencies that are built with Meson, specify USE_MESON_LIBS=1 in
 your NMake command line.
 
 Note that $(VSVER) refers to 14 for Visual Studio 2015 and 15 for Visual
-Studio 2017.
+Studio 2017 and 16 for Visual Studio 2019.  Note that it is recommended
+and possibly required to build glibmm and anything that depends on glibmm
+with the same Visual Studio version, even for Visual Studio 2015, 2017 and
+2019 which Microsoft tried very hard to make the binaries API and ABI
+compatible between these versions, so now the build creates different
+DLL and .lib filenames to make this distinction clear.  If you have
+previously built glibmm with Visual Studio 2017 or 2019 and had DLL and
+.lib filenames in the form of xxx-vc140(-d)-y_z, you may choose to pass
+`USE_COMPAT_LIBS=1` in your NMake build commandline to continue to build
+with the resulting DLL and .lib names be in the former form, but please
+note that this is not recommended unless rebuilding dependent code is
+not convenient.
 
 A 'tests' target will build the test programs for glibmm and giomm, an
 'install' target is provided to copy the built DLLs and LIBs, along with