maint: allow gettext 0.17 again

maint: allow gettext 0.17 again

Commit 041c9c47 traded the 'gettext' module for the lighter 'gettext-h'
module, so as to not require the latest gettext release (we only need
the latest release if we ship gettext as a dependent library, but
coreutils has long preferred to use it as an external library).
But that commit overlooked two places necessary to allow the use of
gettext 0.17.

This does not force you to downgrade (using gettext 0.18.1.1 is still
just fine), nor does it affect tarballs (once a tarball is built
with a given gettext version, it can be built on other machines
regardless of what gettext version is present).

* bootstrap.conf (buildreq): Relax prerequisite.
* configure.ac (AM_GNU_GETTEXT_VERSION): Likewise.