Better linking with custom libzmq DLLs

The official ZeroMQ windows installable creates DLLs like

```
libzmq-v100-mt-3_2_3
```

Added a new project setting called "LibZmqDllName" so that users can specify the name of the libzmq dll rather than just force "libzmq.dll". This makes it simpler to manage multiple libzmq DLLs floating around the system. Usage looks like:

```
%SYSTEMROOT%\Microsoft.NET\Framework\v4.0.30319\msbuild build.proj /p:UseCustomLibzmq=true /p:LibZmqDllName=libzmq-v100-mt-3_2_3
```