simplify make catalogue

[boeschf]

Current state:

• unit tests and an example build their own mechanism catalogue
• repeated logic for detecting GPUs, passing compile time flags

Proposal:

• we should leverage the cmake variables and targets which are already there since these catalogues are internal to arbor
  => build a static library with mechanisms, which links to arbor-private-deps

[thorstenhater]

I like, simplification and a bit of isolation!