POSIX lesson please

[Hal Murray]

Do we require support for timer_create?  Should we?

Context is maxOS doesn't support it.
  macOS build fails because timer_create is missing (#528)
  https://gitlab.com/NTPsec/ntpsec/issues/528

Fedora man page says:
       timer_create - create a POSIX per-process timer
       #include <signal.h>
       #include <time.h>
       Link with -lrt.
       timer_create(): _POSIX_C_SOURCE >= 199309L
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

devel/hacking.txt says:
  Only POSIX-1.2001/SUSv3 library functions should be used

You can see POSIX.1-2001, SUSv3, online for free here:
    http://www.unix-systems.org/version3/

http://www.unix.org/version3/inttables.pdf has a table that says "tmr" for the 
timer_xxx functions under the POSIX Base column.  setitimer says xsi in that 
column.  I assume those are tags for some sort of subset, but I haven't found 
the descriptions for them and/or what POSIX requires or what we require.


-- 
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