Use host to generate miniupnpcstring.h, instead of wine

Simplifies gitian win32 build by removing the wine build-time dependency.  Patch by @sipa, with cleanup and testing by wtogami.  Resulting binaries seem to operate just fine.

Getting rid of wine makes it much easier to use gitan LXC.  After gitian LXC is fixed, it will be possible to do deterministic gitian builds wthin Linux VM's, making the process more accessible to a greater number of people.