cmake: make pre-built library discoverable via CMake configuration files

[ConnorBaker]

Initial work, started by @SomeoneSerge to add Mitsuba 3 to Nixpkgs: NixOS/nixpkgs#269378

This PR adds the ability to make nanothread discoverable via CMake instead of requiring projects vendor it in-tree.

Additionally, it allows testing through CMake by using CTests

[ConnorBaker]

I force-pushed to update the PR and take care of the extra $CMAKE_CURRENT_SOURCE_DIR I introduced -- thank you for the feedback!

[wjakob]

A few more questions/followups (sorry, I'm not so familiar with the CMake install workflow)

[ConnorBaker]

After reading through the CMake docs, I've switched to use CMAKE_INSTALL_* where appropriate -- these should be relative paths, as each has a CMAKE_INSTALL_FULL_* counterpart which is the absolute path.