mdate-sh: ignore $SOURCE_DATE_EPOCH (again). * lib/mdate-sh: pay no attention to SOURCE_DATE_EPOCH. The previous change (2025-05-15) was a misunderstanding; looking at SOURCE_DATE_EPOCH causes spurious makeinfo rebuilds: https://lists.gnu.org/archive/html/automake/2025-06/msg00021.html * t/mdate5.sh: remove check for this. * t/txinfo-vtexi4.sh: likewise. * doc/automake.texi (Texinfo): remove mention of this. * NEWS: update.