This reduces download times significantly as downloaded decreases
from over 1 GB to around 60 MB.

Origin is https://github.com/jedbrown/cmake-modules.

This module is automatically checked out when OGS_BUILD_GUI=ON and the executable
gets packaged with the Data Explorer.

Tetgen source is hosted at https://github.com/ufz/tetgen.git

This will prevent Travis-builds from failing because of the bad
availability of the original git repo.