Have `-Wformat' better recognize the `gettext' procedure.

Have `-Wformat' better recognize the `gettext' procedure.

Fixes <http://bugs.gnu.org/10846>.
Reported by Bruno Haible <bruno@clisp.org>.

* module/language/tree-il/analyze.scm (gettext?): New procedure.
  (const-fmt): Add `env' parameter; update callers.  Use `gettext?'.
  (format-analysis)[check-simple-format-args]: Actually support
  gettextized format strings.

* test-suite/tests/tree-il.test ("warnings")["format"]("non-literal
  format string using gettext"): Use `gettext' as the procedure name.
  ("non-literal format string using gettext as _"): New test.
  ["simple-format"]("unsupported, gettext"): New test.