Use __APPLE__ instead of MACOS

This appears to be the standard check for an Apple OSX system.  I have
no idea why MACOS appears to sometimes work and sometimes not work,
but __APPLE__ always seems to work.
Don't add main() to shared libraries

This just seems like a bad idea to me, but I guess we'll have to see
if it actually works...  main() was failing some linkage on OSX, which
may be because we don't want main() in a shared library, but may also
be because I missed something inside of jrbgui...

I guess we'll just have to see!
Have everything depend on Port{Audio,MIDI}

It appears everything should link against these, and the pkg-config
files already do so.
Use <> for system includes

This is correct, "" is incorrect.