Add a folder for building in

Achieves 3 things:
- a folder for Eclipse project to build in, allows to provide a cleaner Eclipse .project / .cproject
- a folder for automake users to build in
- build artefacts won't show up in git as new files all the time implying changes where there aren't any

Since .gitignore is in the folder, it isn't empty.

Since everything but .gitignore is ignored, nothing in it will ever show up.
Ah, I missed that this is not the usual .gitignore but that it is in the build directory. Now I get it :)