Rules to cross-compile win32 on Linux

SCons rules to cross-compile using the MinGW tools on Linux. Need to install the `mingw` packages.

Uses the SCons tool posted on [the SCons wiki](http://www.scons.org/wiki/CrossCompilingMingw). 

Need to adapt it to 64 bit as well.

TODO:

- [x] Fix Travis build script with new SCons rules