its: Fix the value of the xml:lang attribute inserted by msgfmt.

its: Fix the value of the xml:lang attribute inserted by msgfmt.

Reported by Matthias Klumpp <matthias@tenstral.net>
at <https://savannah.gnu.org/bugs/?64025>.

* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add bcp47.
* gettext-tools/src/its.c: Include "bcp47.h".
(its_merge_context_merge_node): Convert the language / locale name to BCP 47
syntax before storing it in the xml:lang attribute.
* gettext-tools/tests/msgfmt-xml-3: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
* NEWS: Mention the change.