Add native MIDI support for Mac OSX via Core MIDI API

[HunterZ]

A couple of years ago I contributed native MIDI support for Windows and Linux, but was not able to do so for Mac OSX because I lacked a workable development environment. This has been remedied, and I was able to implement and test Core MIDI support in an OSX Mojave VM talking to a real Roland SC-88 via a USB-MIDI interface.

As with Windows, support for this is automatically enabled in OSX builds because it's an OS-provided functionality.

File change notes:

• CMakeLists.txt:
  • link required Core API libraries on OSX
• src/MusicSrc/MusicDevice.c:
  • include relevant headers for native MIDI support on OSX
  • add Core MIDI handles to native MIDI device struct on OSX
  • change Windows NativeMidiSendMessage() to take a pointer for outHandle so that calling code can stay common between that and OSX
  • add device name lookup utilities from rtmidi library
  • implement Core MIDI based NativeMidiSendMessage() function
  • implement Core MIDI device init/destroy and device query logic
  • update NativeMidiSendMessage() callers to pass outHandle as pointer for Windows and OSX

[MaddTheSane]

You can see what I meant with my own pull request to your pull request: https://github.com/HunterZ/systemshock/pull/2

[HunterZ]

You can see what I meant with my own pull request to your pull request: HunterZ#2

Thanks. I don't know much about coremidi and was just going off of what examples I could find, so I'm happy to merge in your suggestions.