fpmagic looks quite interesting -- very barebones, but at least its a start!
Footwork too looks like a neat project and I especially think the closer connection between code and interaction is beneficial. It is the same thing which impresses me about libfive and the whole stack around it: it is not just a GUI, not just a scripting language, not just an API -- it is all at once!
From my research into available libraries for geometric constraint solving the best choice is probably SolveSpace. It is actively developed and fast. However, I haven't yet put action behind my thoughts and I'm willing to try things out and make some progress.