Further simplify configure-time locale testing

* configure.ac (gl_STDLIB_H): Replace this, not gt_LOCALE_FR_UTF8,
as this lets us also omit MB_CUR_MAX checking that will never be
executed, and also lets us remove testing for ecvt, fcvt, gcvt
that Emacs doesn’t need.
(AC_C_RESTRICT): Invoke explicitly, as Emacs uses ‘restrict’
directly.  Also, simplifies the replacement for gl_STDLIB_H, which
would otherwise need to AC_REQUIRE AC_C_RESTRICT.
* lib/gnulib.mk.in: Regenerate via admin/merge-gnulib.

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