The GEOS source tree on GitHub (​https://github.com/OSGeo/geos) is a mirror of the Subversion repository (​https://svn.osgeo.org/geos/).

To get the source from GitHub, and start a configure:

git clone https://github.com/OSGeo/geos.git
cd geos
./autogen.sh
./configure

To fork GEOS to GitHub to modify and test new features, follow the guideline ​Fork A Repo.