1. Fix the path to eigen, now that it uses gitlab instead of bitbucket.
2. Remove an unrecognized compiler option.
3. Remove an obsolete benchmark.

This CL only unbreaks the build, it is likely that it is still not
at par with the cmake build.

https://github.com/ceres-solver/ceres-solver/issues/628

Change-Id: I470209cbb48b6a4f499564a86b52436e0c8d98ef