cross compilation to windows gets wrong TMsg size

[AvdN]

The current installer for windows is for zmq 4.0.4 which has 32 byte TMsg size.
When cross compiling (-d:mingw) the size of TMsg is determined compile time based on the locally installed zmq (on Linux, 4.2.0) which is 64 byte.

It should be possible to set the target zmq version number and/or set the TMsg size explicitly

This could be done from the commandline (-d..) or if possible from the importing .nim module (when compiled with -d:mingw)