The GEOS source tree on !GitLab (https://gitlab.com/geos/libgeos) is a mirror of the Subversion repository (https://svn.osgeo.org/geos/).

To get the source from !GitLab, and start a configure:
{{{
git clone https://gitlab.com/geos/libgeos.git
cd libgeos
./autogen.sh
./configure
}}}