Historically, we had configure.in in the tree, and generated configure
with autoconf via client.mk at build time, and we'd run that configure.
As a side effect, configure would be shipped in source tarballs, as with
traditional autoconf-driven software.

Bug 1671424 changed that, and made `mach build`/`mach configure` invoke
configure.py directly, without needing configure. As a side effect,
configure stopped being generated in source directories. An unexpected
side effect was that configure stopped being shipped in source tarballs.

Anyways, it has been long enough now that adding a configure file in the
tree shouldn't cause too many problems with people updating their trees
where a configure file might exist from those older builds.

Differential Revision: https://phabricator.services.mozilla.com/D155923