"interesting" changes - please test

[Hal Murray]

I just pushed various cleanups to wscript and friends.  This started as a 
simple idea to make it easy to add sanity checks to get sensible error 
messages from waf configure rather than obscure error messages from the c 
compiler.

The pipeline worked, and I've got it running on various machines, but I can't 
test Apple or Solaris (and probably others).

The first step was to fix the test for timer_create so it worked.  Since that 
had never been tested before, I also had to fix the seccomp list in 
ntp_sandbox.

But timer_create is included in POSIX, so I think we get to rip out a bunch of 
ifdefs.  I haven't done that yet.

I also tried to set things up so that it didn't drag in libpthread if built 
without DNS.  More work needed.

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