unbundle dependencies

[minrk]

Comment:

gmsh is currently using bundled versions of several dependencies:

Found Eigen[contrib]
Found ANN[contrib]
Found ALGLIB[contrib]
Found MathEx[contrib]
Found TinyXML2[contrib]
Found Metis[contrib]
Found Voro++[contrib]

At least Eigen, ANN, TinyXML2, Metis, and Voro appear to be packaged on conda-forge already (Voro not on Windows yet conda-forge/voro-feedstock#3). Probably most of these should be added to host dependencies.