build: reinstate distribution of man pages

build: reinstate distribution of man pages

man pages change little between systems,
so falling back to distributed pages make sense
when cross compiling or lacking perl.

* man/local.mk: Add all man pages to EXTRA_DIST
so that they're distributed in the generated tarball.
Use the dummy-man page generator if cross compiling.
Set TZ to avoid a distcheck failure where man pages
used a diffent month than those rebuilt (with a .timestamp).
* man/dummy-man: Only fall back to generating a stub
if copying an existing man page fails.
* man/help2man: Sync portable TZ=UTC0 specification
from upstream help2man.
* NEWS: Mention the build-related change.
Fixes https://bugs.gnu.org/28574