Use Boost.Config for platform dependent macros, CMake for building.

Hi there,

**Introduction:**
I've been following this project for some time now, but never had the guts to really dive into the code.

Long time ago, I was seriously involved in nwn and (a bit less) in nwn2, creating and managing online modules, but also dissecting the data files and writing tools. I always wanted to start such a re-implementation project but never had the time.

My plan is to look what can be improved on the OpenGL part, as you mentioned that you would need some help on this.

**About this PR:**
These changes are not about that at all, and I'm perfectly aware that they are arguably useful (especially the CMake one if you like autotools). It was some kind of an introduction into xoreos code for me, trying to understand how things are hinged together.

**About the CMake change:** My opinion is that it is a better build system than autotools and I've never been able to grasp autotool syntax.

**About the other changes:** I think that the less checks a project needs the more portable it will be. Therefore, I tried to replace some platform dependent checks with the Boost implementation, that I feel is trustworthy. And as the project already depends on Boost, it's probably OK to use these as well.