Recently introduced build error

[Eric S. Raymond]

Hal Murray <hmurray at megapathdsl.net>:
> [ 63/113] Compiling libntp/msyslog.c
> In file included from /usr/include/python2.7/pyconfig.h:6:0,
>                  from /usr/include/python2.7/Python.h:8,
>                  from ../../libntp/pymodule.c:7:
> /usr/include/python2.7/pyconfig-64.h:1199:0: warning: "_POSIX_C_SOURCE" 
> redefined
>  #define _POSIX_C_SOURCE 200112L
> 
> <command-line>:0:0: note: this is the location of the previous definition
> 
> That is on Fedora 26

You are in a maze of twisty little compiler quirks, all different.

What I was trying to do is reduce the scope of our dependence on GNU C
features. So I changed the global options, removing -D_GNU_SOURCE and
replacing it with -D_POSIX_C_SOURCE=200809L -D_XOPEN_SOURCE=600.

This worked just fine on my Ubuntu Linux, gcc 5.4.0.  It wreaked
different kinds of havoc on other systems.  Sigh...commit reverted.
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