Allow using system ceres

[Tobias-Fischer]

Feature

Currently gtsam always ships its own bundled version of ceres. It would be great if a similar approach as for Eigen or Metis is taken, where alternatively the system version can be used.

Motivation

Bundling ceres is undesirable if package managers like conda-forge are used. Also, most distributions ship ceres themselves, so there should not be a need for the bundled version.

Pitch

Create a cmake/HandleCeres.cmake file that follows the structure of HandleMetis.cmake/HandleEigen.cmake and allows defining GTSAM_USE_SYSTEM_CERES if one wants to use the system ceres.

Alternatives

N/A

Additional context

This relates to our efforts to package gtsam in conda-forge. Currently we patch gtsam to always use the ceres shipped with conda-forge (https://github.com/conda-forge/gtsam-feedstock/blob/main/recipe/unvendor-ceres.patch)

[varunagrawal]

@Tobias-Fischer I believe this is already tracked in #292? Would you be okay with closing this and updating that issue so we can track all the system libraries together?

[Tobias-Fischer]

Indeed this issue already mentions Ceres, so happy to close here.