The PostGIS source tree on OSGeo ​Gigs instance (​https://git.osgeo.org/_gogs_/postgis/postgis) is a mirror of the official repository.

To get the source from !OSGeo Git Service, and start a configure:

git clone https://git.osgeo.org/_gogs_/postgis/postgis.git
cd postgis
./autogen.sh
./configure

To fork PostGIS to modify and test new features, just click on the Fork button on top of the postgis page. You'll need to login using your OSGeo Userid (a link to register a new account will be found on the login page).