Pluginval as dependency

[sudara]

Opening as a draft for feedback and iteration.

Addresses #137

This improves support for pluginval as a CMake sub-project in a JUCE project.

• Moves JUCE from submodule to CPM dependency on 8.0.4
• Only adds JUCE when top level
• Only sets deployment target when top level
• Only generate docs when top level
• Removes PLUGINVAL_FETCH_JUCE option (no longer needed, superseded by not being top level)

Open Questions:

• Do we need to enforce minimum JUCE version for this use case?

Remaining TODO:

• Get example plugins building in CI

---

Note to Dave and other maintainers:

With CPM, it's always a good idea to have the environment variable CPM_SOURCE_CACHE set — this will let you do offline builds vs. hitting the network on reconfigure. We can also add this optimization in CI too.

Add the following to .bashrc or .zshrc:

export CPM_SOURCE_CACHE=$HOME/.cache/CPM

The submodule is gone, so you can optionally clean up your .git/config folder, you can run:

rm -rf .git/modules/modules/juce 

[drowaudio]

Am I supposed to be reviewing or merging this?
I like the feature set but it seems to have not changed much in the last year? Is it stable enough to go in?

You might notice I switched to CPM for a few other dependancies on develop. Can this branch be pulled in line with that?

[sudara]

@drowaudio I've been using this branch for a year as a dependency of my synth but never got the example plugins running in CI. I'll need to get that going and resolve recent conflicts before it's mergable.

[drowaudio]

Ah ok. Poke me if/when you get to that.

I might switch to juce via CPM at some point soon anyway...