An insolated version of the geometric constraint solver from SolveSpace: libslvs
See http://solvespace.com and https://github.com/solvespace/solvespace
The original library code is not independent of the complete SolveSpace cad application, which is somewhat inconvenient if all you need is the constraint solver.
This repository only contains the neccessary code, and is thus quite small. As far as I know, this is the only open source working geometrical constraint solver out there.
Use CMake to create a build system, or IDE project files.
The library itself is within the folder libslvs, which has its own CMakeList.txt file On top level the CMakeList.txt is only an example on how to use the library from an application.
SolveSpace and thus SolveSpaceLib is distributed under the terms of the GPL3 license.