Use even more hackish way to support thousand separators.

Use even more hackish way to support thousand separators.

Seems that in addition on Windows and DOS, also OpenBSD
lacks support for %'d style printf() format strings.
So far that is the only modern POSIX-like system I know
with this problem, but after this hack, the thousand
separator shouldn't be a problem on any system.

Maybe testing if a format string like %'d produces
reasonable output is invoking undefined behavior on some
systems, but so far all the problematic systems I've tried
just print the raw format string (e.g. %'d prints 'd).

Maybe Autoconf test would have been better, but this
hack works also for cross-compilation, and avoids
recompilation in case the system libc starts to support
the thousand separator.