The PostGIS source tree on !GitHub (https://github.com/postgis/postgis) is a mirror of the Subversion repository (http://svn.osgeo.org/postgis/).

To get the source from !GitHub, and start a configure:
{{{
git clone https://github.com/postgis/postgis.git
cd postgis
./autogen.sh
./configure
}}}
To fork PostGIS to !GitHub to modify and test new features, follow the guideline [https://help.github.com/articles/fork-a-repo Fork A Repo].