Bug 1574761 - Avoid race condition creating old-configure. r?build

Bug 1574761 - Avoid race condition creating old-configure. r?build

This is not something that happens under normal circumstances, but it
can happen when you go fancy and run multiple configures in parallel
with different objdirs, and old-configure doesn't exist in the first
place ; then one configure may overwrite old-configure while another is
starting to execute it, resulting in the latter nor executing
old-configure completely.

Differential Revision: https://phabricator.services.mozilla.com/D42428

Gbp-Pq: Topic fixes
Gbp-Pq: Name Bug-1574761-Avoid-race-condition-creating-old-config.patch