Add a custom `$_bootstrap` variable in the PKGBUILD to allow bootstrapping / rebuilding the package after soname changes

Upstream (autotools) bootstrap scripts used to prepare the `wget` and the `gnulib` submodule sources both require `wget` to download `po` (translation) files.
This implies that the `wget` package *(make-)*depends on itself.

While this shouldn't be a problem for regular version bumps, it can lead to issues when bootstrapping the package (e.g. when adding new architectures) and it can have theoretical implicati ons when rebuilding the package in staging due to soname changes (in case `wget` doesn't work until rebuilding for instance).
To address this, the PKGBUILD now contains a custom `$_bootstrap` variable whose value can be modified as needed:

- When set to `0`, the package is built "normally"
- When set to `1`, the package is built by skipping the `po` files download step so the `wget` make-dependency is unused (allowing to bootstrap it or rebuild it after soname changes withou t issue).

**WARNING:** Building the package with `$_bootstrap=1` therefore results in a `wget` package that does not include po (translation) files, so it should still be rebuilt it with `$_bootstrap=0` when the bootstrapping / rebuilding after soname changes is done.

Switch license to SPDX identifier, add perl as an make dependency, switch source to git.savannah.gnu.org
>
> Update license ID to GPL-3.0-or-later
> Add perl as an explicit make dependency (required to generate the man page)
> Switch source to git.svannah.gnu.org (which is the canonical source for GNU software).

(not bumping pkgrel just yet as some more work is being done)

archlinux/devtools!109 (comment 74475)