Bug#992813: postgis: please migrate away from autoconf2.13

[Chris Hofstaedtler]

Source: postgis
Version: 3.1.3+dfsg-1

Dear Maintainers,

your package build-depends on autoconf2.13, which is obsolete and
orphaned in Debian (see #977321).

Please investigate migrating away from autoconf2.13, possibly to
autoconf 2.69 or newer (package name "autoconf").

A quick test rebuild with autoconf2.13 dropped from Build-Depends
appears to work. I could not find an upstream commit since 2016 that
hinted at an autoconf upgrade, so maybe it has happened even
earlier.

Please give it a try :-)

Thanks,
Chris