Threads, stdatomic, old old systems

[Hal Murray]

> Are we dropping the mandate in devel/hacking.adoc to -std=c99 strict
> except for anonymous unions?

If the compiler on the oldest system we run on supports c11, then updating 
that line seems reasonable.

We have a lot of statistical counters that get bumped with ++ or +=n.  I'd 
like to switch them to use a macro that uses stdatomic.h.  If we want to 
support systems that don't support stdatomic, we can hack the macros to do 
it the old/current way.



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