maint: use gnulib's gettext-h, not the gettext module

maint: use gnulib's gettext-h, not the gettext module

* bootstrap.conf (gnulib_modules): Use gnulib's gettext-h, not the
gettext module.  Not only is gettext-h far smaller (it has far fewer
dependencies than the gnulib module), but it does not suffer from
the problem with the gettext module whereby it adds a -I.../intl
option to compilation flags.  That can provoke warnings, since we
don't have such a directory.  We used to work around that via a
hack in bootstrap, but that was ugly and inefficient.