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.)
