The asset 'agmaint-1.3.5.tar.gz' is an AutoTools distribution tarball.
The sources are available on this page.
To perform an AutoTools install please refer either to the README file on the first GitHub page of the project or to the installation section on the wiki, (link below).
Downstream native packages are available for Debian, Raspbian and Ubuntu. Installation instructions can be found on the project wiki at:-
https://github.com/m-grant-prg/agmaint/wiki