Don't update pdfsig.pot when POT-Creation-date is the only change
Everytime we perform a Poppler build the pdfsig.pot
file is
updated with just a new timestamp in POT-Creation-date
field.
That is a nuance as we have to always discard that change in Git prior to submitting a MR or just switching branches locally.
Fix it by only updating the pdfsig.pot
file when there are
changes other than the POT-Creation-date
, because there's no way
to stop xgettext from updating the timestamp.
Some related info: https://savannah.gnu.org/bugs/?49654#comment1
Fixes #1479 (closed)