_XOPEN_SOURCE in ntpd/refclock_gpsd.c => warnings on BSD

[Hal Murray]

> I see no changes related to _XOPEN_SOURCE since 2017.  Perhaps you're
> thinking of GPSD, where there was bunch of rework in that area just before
> the 3.19 release.  

Thanks.  You are probably right.

Eric:  This area just got more complicated.  See #614
strerror_r() has two modes depending on
           The XSI-compliant version is provided if:
           (_POSIX_C_SOURCE >= 200112L) && !  _GNU_SOURCE
           Otherwise, the GNU-specific version is provided.
Our code is (sometimes?) assuming the wrong one.  The GNU version (sometimes?) 
doesn't put the string into the buffer we are printing.




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