I'd like Eigen to be a submodule,
but their repo in is mercurial,
and they do not have an official git mirror.

Maybe I'll spend some time to add a tracker,
but there seems to be problems with the conversion.

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>