cmake submodules inclusion of rtaudio rtmidi

[hyper-temporal]

the stk project includes rtaudio and rtmidi as files. I was just wondering why while these two being submodules, they don't seem to be included that way

[garyscavone]

It is possible that RtAudio and RtMidi could be included as submodules, however, I think there are a few issues, including:

• the .h and .cpp files would be in different directories and thus inclusion in other STK headers and projects would need to be revised;
• it is possible that API updates could be made to RtAudio and RtMidi that would break STK

Thus, my modus operandi is to finalize new releases of RtAudio and RtMidi first, then update those in STK for a new release.