Code review comment for lp://staging/~alan-griffiths/mir/fix-1695221

Hmm. Looks like a clang issue on artful and zesty...

https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=clang,platform=mesa,release=zesty/4718/console

09:57:51 /usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/atomic:236: error: undefined reference to '__atomic_load_8'
09:57:51 /usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/atomic:225: error: undefined reference to '__atomic_store_8'
09:57:51 clang: error: linker command failed with exit code 1 (use -v to see invocation)
09:57:51 examples/CMakeFiles/mir_demo_client_eglsquare.dir/build.make:114: recipe for target 'bin/mir_demo_client_eglsquare.bin' failed

https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=clang,platform=mesa,release=artful/4718/console

09:55:32 make[3]: Leaving directory '/<<BUILDDIR>>/mir-0.27.0+artful4805bzr4185/obj-x86_64-linux-gnu'
09:55:32 /usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/atomic:236: error: undefined reference to '__atomic_load_8'
09:55:32 /usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/atomic:225: error: undefined reference to '__atomic_store_8'