bug#19376: [PATCH 4/4] build: use -pi.bak instead of -pi

[Assaf Gordon]

tags 19376 wontfix
close 19376
stop

(triaging old bugs)

As explained in https://bugs.gnu.org/19377#18 ,
Leaving backup files scattered is undesired.

Closing this as "wontfix".

regards,
 - assaf