Quite a messy commit, hopefully new ones should be shorter

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>

Rewrote cmake, now a bit cleaner, checks for compiler flags.

Added Eigen3, v3.2.8 in case it's not found on the system.

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>

Also add relative (optional) cmake target: "docs"
Documentation is still incomplete, needs more
work.

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>