maint: install minimal supported autoconf through convenience targets

maint: install minimal supported autoconf through convenience targets

To make testing of Automake in conjunction with it easier and more
reproducible.  It might be nice to also have convenience targets to
actually automatically run the Automake testsuite with such autoconf
version, but that is left to later patches.

* configure.ac: AC_SUBST the definition of $required_autoconf_version.
* maint.mk (WGET): Move definition earlier.
(gnu-ftp, ac-v, ac-n, ac-p, ac-t, ac-l, ac-d): New auxiliary variables.
(fetch-minimal-autoconf, build-minimal-autoconf): New convenience
targets.
* .gitignore: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>