Detect system-installed Rt* libraries

[OPNA2608]

Closes #185.

• RtAudio & RtMidi: Uses pkg-config in CMake to find these libraries & required settings, falls back to finding the libraries & header directories via CMake-native mechanisms (find_library & find_path)
• RtAudio: Checks if the found headers have the required function RtAudio::getCompiledApiName (introduced with 5.1.0), rejects the detected results if they don't
• JACK & PulseAudio: Fallback if pkg-config is not available
• JACK: Also detect on Darwin

Builds & runs on Ubuntu 18.04 with only RtAudio checked out (RtAudio is still version 5.0.0 there, so only system-RtMidi can be used). I'll test this on Darwin & a more updated Linux some time this week.

Do you want a CMake option to control this detection behaviour? If yes, what should the default value be?

[Wohlstand]

The detection would be fine to be the default, and then have both options:

• fallback into a shipped build of the library
• the manually-set flag that will enforce use the shipped build of the library always

[Wohlstand]

P.S. The conflict here is because I had to comment out the code that enables the ASIO support on Windows, that doesn't work properly, unfortunately, and people complained me several problems with it:

• just no audio
• startup crash

So, I desired to disable this at all. At the same time, this is a proprietary technology that is not met by the FOSS community.

[OPNA2608]

• Rebased to fix merge conflict
• Added USE_VENDORED_RTMIDI and USE_VENDORED_RTAUDIO CMake options to disable the library detection (default value is OFF)

[Wohlstand]

Good! :fox_face: :+1:

[OPNA2608]

Should I port this to OPN2BankEditor or will you do that?

[Wohlstand]

If you want, feel free to try :fox_face: Also please apply my patch to disable the ASIO on the OPN2 side too, it still being enabled here.