Update mingw32 cross-compile instructions
The INSTALL.mingw file is now actually a script that can be passed to bash. It will download and compile the dependencies, and then build the pidgin-otr NSIS installer and zip file. Note that the GTK and pidgin libraries are downloaded in Windows binary form (though their checksums are checked), and not built from source. This script is known to work on one particular Ubuntu 14.04 machine. It would be cool if others could try it as well, and it would be super-cool if we could turn this into a deterministic build.