Allow using system pybind11

[sanjayankur31]

Hello,

Would it be possible to allow the usage of system installations of pybind11 instead of the included submodule please? For downstream distributions, that would make packaging arbor a lot easier.

(We're working on including arbor in NeuroFedora, and that's where this came up. The tracker ticket is here)

Cheers!

[bcumming]

Thanks for the heads up.
Pybind11 is header only, so do we need to rely on the system version?

[sanjayankur31]

We can continue to unbundle it downstream, but it would be nice to have a simple way of switching to system versions (of all dependencies in general).

[schmitts]

Cf. #1198

[brenthuisman]

Sebastians PR implemented this.