Build fails with -DARB_USE_BUNDLED_UNITS=OFF because the install(units) statement is unconditional

[yurivict]

Please allow arbor to cleanly build with an externally installed units library.

[thorstenhater]

Currently I cannot find a Spack or brew package for llnl::units so we rely on the vendored one.
That'll change if and when such packages arrive. Does BSD provide such a package?

[yurivict]

Yes, I've added the port for unit yesterday to FreeBSD.

The correct algorithm would be to first look for the installed unit using find_package, use it if it exists, and fall back to the bundled one if not.

[thorstenhater]

Thanks, you are one of the first packagers then do so and I'll happily add the scaffolding!

[thorstenhater]

I don't know if you have the bandwidth, but if you could test with the linked PR, I'd be grateful to see the results.

[yurivict]

There is something missing in the PR, I left the comment there.

[thorstenhater]

Hi @yurivict,

this took I while longer than I expected it to do, but that was due to me going off the rails and retiring
our whole cursed sub-module construction. Main now uses CPM --- for better or worse --- and should
prefer installed packages over internal ones. Missing packages will be pulled in from external.

See here on how to avoid that:
https://github.com/cpm-cmake/CPM.cmake?tab=readme-ov-file#cpm_use_local_packages

I also tweaked the installation of units to install iff units was built along side Arbor.