Solution: Use CMD.EXE environment variable to extract DevStudio version
number and build using it. This even supports machines with multiple
DevStudio versions installed, as long as the build for each version is
done on a separate window with the correct environment.
If multiple version builds are desired from a single CMD.EXE, edit the
buildall.bat file to uncomment the build statements for each specific
version desired.

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.)