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>

README

@@ -63,7 +63,6 @@ provided by your OS distributor:
     * bridge-utils package (/sbin/brctl)
     * iproute package (/sbin/ip)
     * GNU bison and GNU flex
-    * GNU gettext
     * ACPI ASL compiler (iasl)
 
 In addition to the above there are a number of optional build