BRL-CAD has a long history of pithy attempts at constraint solving.

There is a relatively simple forward-kinematic solver that was
implemented as a GED command (see 'joint' command).  It utilizes
'grip' objects and outboard text files that describe constraint
relationships.  It remained broken and unused for about 20 years until
it was fixed in 2014.  It surprisingly works, and seems to work well
solving linear relationships (3D geometric constraints).

There is a relatively new system that utilizes 'joint' objects,
currently creating a 6DoF relationship between objects but with the
intention of expanding it into supporting other (more constrained)
joint types like hinges, pivots, saddles, etc.  This system is
integrated into MGED/Archer and is interactively editable with mouse
controls.

The scope of libpc is to serve as a wrapper over a geometric
constraint evaluation system with the likely expectation that one or
more 3rd-party solver will be incorporated to solve parametric
constraints and joint articulation.  The intention and target purpose
is for librt to rely on this library for data types and evaluation.


GECODE
http://www.gecode.org
mit

GLOP
https://developers.google.com/optimization
apache2