We can already develop against beta version of the new autoconf (current beta is 2.72c, but final will be 2.73, if I understood correctly)
Depends on #47830
Note that even when we make the code change to configure.ac, our release tarballs will not benefit from it until we create those tarballs with a new enough autoconf (2.73).
Patch attached for all branches, including S2_6
autoconf-2.73 (of which a beta was just released) provides AC_SYS_YEAR2038 for resolving year 2038 problems. We probably need to take that to use (conditionally, not breaking build with older autoconf versions)