Add support for cross compiling from linux for windows (only header-only libraries supported)

[R1tschY]

Fixes #14 . Fixes the problem for header-only libraries only.

[Manu343726]

Thanks for your work, cross compiling was in the roadmap since the very beginning, but I had no time enough to check it out. I will merge this even if the ci tests are not passing. The current travis setup bootstraps builds for the examples by committing and pushing to their repos, so you need credentials (Git user, email, and github password) in order to run the builds. I should update the docs, sending PRs to a develop branch would allow me to run tests prior merging to master.

Also note merging does not push changes to biicode cloud. I will be much more conservative here since changing the Boost scripts may break a lot of user code. OTOH your changes are simple and shouldn't break anything.

[Manu343726]

Only one question: What means it works for header only libs only? In that cases you don't actually build anything from Boost, so what's the point of cross-compiling?

[R1tschY]

Only one question: What means it works for header only libs only? In that cases you don't actually build anything from Boost, so what's the point of cross-compiling?

When you cross compile a application, but you only use a header-only boost library (e.x. signals2), this pull request is needed to compile. The reason is that bootstrapping is also done when no boost library must be compiled. I added #17, but its a more complicated pull request so I separated it in two pull requests.