maint: avoid spurious "make distcheck" failure

maint: avoid spurious "make distcheck" failure

When the generated file, doc/constants.texi, happens to be older than
doc/coreutils.info, it will not be updated until/unless its generated
contents change.  This is due to way that rule is careful to update
the file, to avoid provoking a pointless rerunning of makeinfo.

Note that this does not happen when one first runs "make distclean",
as recommended in README-release.  However, I sometimes run it as
a more-rigorous "make check", and shouldn't have to manually run
"make distclean" first, in that case.

Before this change, one could reproduce the failure by running
`touch -dyesterday doc/constants.texi && make distcheck`.  It would
fail with "makeinfo: could not open ../../doc/coreutils.info-t
for writing: Permission denied"
* Makefile.am (dist-hook): Touch the two generated files, so that
they cannot be out of date wrt doc/coreutils.texi.