Also, document this stuff better. Suggested by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2021-07/msg00011.html
* doc/year2038.texi: Document --disable-year2038.
* m4/year2038.m4 (gl_YEAR2038_BODY): Support ‘./configure
--disable-year2038’ to disable 64-bit time_t when that is not the
default. Arg is now either empty or nonempty (not a shell
command) and is evaluated at m4 expansion time instead of at
runtime; all callers changed.