(DISTFILES.common): Add remove-potcdate.sin.

(DISTFILES.common): Add remove-potcdate.sin.
(.SUFFIXES): Add .sed and .sin.
(.sin.sed): New rule.
($(DOMAIN).pot-update): Depend on remove-potcdate.sed. If the old
POT file exists and the new POT file differs from it only in the
first POT-Creation-Date line, keep the old POT file. So the PO files
don't need to be remade, and automake's "distcheck" target succeeds.
(mostlyclean): Remove remove-potcdate.sed and additional temporary
files.