Problem: can't build without libsodium from command line (MSVC)

There is an option to enable/disable libsodium via the Visual Studio
UI. This is not practical for command-line usage (via msbuild).

Solution: add configure.bat that searches for libsodium in sibling
directory to libzmq; if it finds it, defines HAVE_LIBSODIUM 1.

This is consistent with zproject, which has the same problem and
is getting the same solution for all dependency resolution on
Windows.

Note that this approach also provides a way to support tweetnacl
via a configure option.

Also, removed duplicate props files and re-copy in configure.bat
as it was an insane nightmare to update these by hand. (and not
clear that they were identical. Now it's forced.)

builds/msvc/platform.hpp

@@ -1,41 +1 @@
-/*
-    Copyright (c) 2007-2016 Contributors as noted in the AUTHORS file
-
-    This file is part of libzmq, the ZeroMQ core engine in C++.
-
-    libzmq is free software; you can redistribute it and/or modify it under
-    the terms of the GNU Lesser General Public License (LGPL) as published
-    by the Free Software Foundation; either version 3 of the License, or
-    (at your option) any later version.
-
-    As a special exception, the Contributors give you permission to link
-    this library with independent modules to produce an executable,
-    regardless of the license terms of these independent modules, and to
-    copy and distribute the resulting executable under terms of your choice,
-    provided that you also meet, for each linked independent module, the
-    terms and conditions of the license of that module. An independent
-    module is a module which is not derived from or based on this library.
-    If you modify this library, you must extend this exception to your
-    version of the library.
-
-    libzmq is distributed in the hope that it will be useful, but WITHOUT
-    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
-    FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
-    License for more details.
-
-    You should have received a copy of the GNU Lesser General Public License
-    along with this program.  If not, see <http://www.gnu.org/licenses/>.
-*/
-
-#ifndef __ZMQ_PLATFORM_HPP_INCLUDED__
-#define __ZMQ_PLATFORM_HPP_INCLUDED__
-
-//  This is the platform definition for the MSVC platform.
-//  As a first step of the build process it is copied to
-//  zmq directory to take place of platform.hpp generated from
-//  platform.hpp.in on platforms supported by GNU autotools.
-//  Place any MSVC-specific definitions here.
-
-#define ZMQ_HAVE_WINDOWS
-
-#endif
+#error "Run configure.bat to create platform.h"