Add atomic to link_libraries - fixes compilation on ArchLinux ARM with GCC8

[Manawyrm]

Hi,

the title already says it all. :)
This fixes the compilation on ArchLinux ARM with modern GCC versions.
Shouldn't have negative side effects on Debian.

Tobias

[juj]

Thanks! Verified that this works on my

pi@raspberrypi3plus:~/code/fbcp-ili9341/build_mz61581 $ uname -a
Linux raspberrypi3plus 4.14.34-v7+ #1110 SMP Mon Apr 16 15:18:51 BST 2018 armv7l GNU/Linux
pi@raspberrypi3plus:~/code/fbcp-ili9341/build_mz61581 $ gcc --version
gcc (Raspbian 6.3.0-18+rpi1+deb9u1) 6.3.0 20170516
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

system that I have at hand on work desk at the moment. I used to run against Raspbian Jessie (GCC 4.9.2), not sure how that behaves with atomic, and cannot test right now, but I'll assume it's safe (should be easy to conditionally exclude if atomic is not a thing there)