tools: Drop gettext as a build dependency

tools: Drop gettext as a build dependency

It has not been a dependency since at least 4.13.  Remove its mandatory check
from ./configure.

Annotate the dependency in the CI dockerfiles, and drop them from CirrusCI and
TravisCI.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>