Update Visual Studio builds for UCDN usage

Update Visual Studio builds for UCDN usage

We now use UCDN by default, so make it so in the build files; however
don't hardcode HAVE_UCDN as one may still opt not to use it (but pass
it in as a CFLAG unless one explicitly disables UCDN by using NO_UCDN=1
on the NMake command line).

Note that we are not blocking builds where UCDN is disabled along with
GLib and ICU, as that will trigger a build error anyways which will tell
the user what needs to be done to remedy this.