Workaround for CMake 3.12 bug passing -thread to nvcc (#649)

CMake wants to run a device link pass with nvcc despite
there being no CUDA seperable compilation enabled anywhere,
and then passes on -pthread to that unnecessary nvcc
invocation when we use the Threads dependency. The latter,
at least, is fixed in CMake 3.13.

We used the prefer -pthread option for compatibility with
our earlier build configuration; turning it off will
hopefully have no consequence.

We also enable device linking on the arbor library. Which
is not needed, but if they are going to insist on doing it,
it should be on the library rather than the executable.

CMake then goes and does it on the executable anyway. Great.

Fixes #645.