build: Bump mimimum required Automake version.

* configure.ac (min_automake_version): Bump to 1.16.3.

The AM_DISTCHECK_DVI_TARGET variable that we now use has been
introduced in Automake 1.16.3.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>