Allow Unity build. Two parts: - missing include guard in solver_utils.h - exclude schur_specializations from unity build. This can only be done with cmake >= 3.16, so only allow unity build for these versions. Cuts build time by 5 minutes with default settings. libceres.a shrinks from 7549968 bytes to 7009696 bytes. Change-Id: I833ac8266623da0e725cc2f07a6415b4fa72bc18
Ceres Solver is an open source C++ library for modeling and solving large, complicated optimization problems. It is a feature rich, mature and performant library which has been used in production at Google since 2010. Ceres Solver can solve two kinds of problems.
Please see ceres-solver.org for more information.