Issue 2828 and building against atomics.

[Matthew Schumacher]

Backstory:
https://github.com/FreeRADIUS/freeradius-server/issues/2828

This patch:
https://github.com/FreeRADIUS/freeradius-server/commit/5a7df325116a3c2e995e5b37d8f77665336f1caf

Fixes a build issue on Debian where it needs to link against atomics to 
compile.  On slackware, that patch also causes the build system to link 
against libatomics, but as that library is in the gcc-5.5.0 tarball, 
slackware includes it in the gcc package thus making the freeradius 
binary require the entire gcc build suite package on this platform.

If I revert the patch, it doesn't link against libatomic and thus I no 
longer have this dependency.  Alan suggested installing libatomic as the 
solution, and while I can make a package for that, I don't understand 
what I would be loosing by omitting -latomic from the linker and 
building without it.  Can someone explain why I shouldn't just revert 
the patch in my build script and eliminate this dependency?

Thanks,
schu