Merge from Gnulib in emacs-28 branch

[Paul Eggert]

To address the usual portability/correctness issues (new and improved 
with C2x looming :-) I merged from current Gnulib into the emacs-28 
branch, and this merge should eventually hit the Emacs master branch. As 
it's been a while since the last merge from Gnulib I expect there will 
be some glitches; please let me know what you run into. You will 
probably need to build from scratch or do a 'make bootstrap'.

One thing in particular: 'configure' now prefers 64-bit to 32-bit time_t 
on platforms that have both. The only known platforms like this are 
glibc 2.34+ on ARM and x86. The upside is that Emacs won't stop working 
after 2038 on these 32-bit platforms; the downside is that there may be 
issues (which we don't yet know about) if Emacs is linked to libraries 
that require time_t to be 32 bits. If you encounter the downsides you 
can configure with --disable-year2038. Please let me know of any 
problems in this area; I have tested this on Fedora 34 x86 client, but 
not on other platforms.

There's a bit more discussion about this merge here:

https://debbugs.gnu.org/bug=50985