tests: correct the "make check"-run check-AUTHORS test

tests: correct the "make check"-run check-AUTHORS test

* src/Makefile.am (check-AUTHORS): Revert back to using
en_US.UTF-8, to ease parsing (English-only) text around the
list of names, even when .po files are not installed.
Reported by Andreas Schwab.
Along the way, use $(AM_V_GEN), not "@".
(sc_tight_scope): Use $(AM_V_GEN) here, too.
* gnulib: Update submodule to latest, for the proper_name_utf8 fix
that makes --version output print the UTF-8 rendering of author names
in more cases.