Attoconf itself is released under the GNU General Public License (GPL).

As a derivative work, this also applies to the 'configure' script shipped
with programs that use the attoconf library. This is a deliberate decision:
while 'configure' is program-specific options, many programs will want
similar options.

However, the actual programs build using attoconf are not derivative works
and thus may be released under any license.

The GPL is available in the file gpl-3.0.txt