Fix for #614

[Hal Murray]

I think fixing the _GNU_SOURCE is going to take a lot of work, and we should 
probably a change of that nature lots of time for extra testing.  So I 
implemented a local wrapper that uses a configure time test.

The code for mystrerror is in the bottom of libntp/msyslog.c
The waf code is in wafhelpers/check_strerror.py

Please feel free to improve them.

The waf code is ugly, but seems to work.
It adds -Werror to CFLAGS to turn a warning into an error, then removes it.

---------

I've tested on Fedora and NetBSD.

My suggestions for testing are to change the mode on your NTS cookies file.  
To not readable.
Default is:
  include/nts.h:#define NTS_COOKIE_KEY_FILE "/var/lib/ntp/nts-keys"
Or wherever you put it via "nts cookie <xxx>" in your ntp.conf

26 Aug 19:22:17 ntpd[20786]: NTSs: can't read old cookie file: 
/var/lib/ntp/nts-keys=>Permission denied


-- 
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