Call Barrier when given the option --adapter or -z to conform
to the protocol of the adapters in
github.com/incf-music/music-adapters.

Fix python3 syntax error

Add function to load boolean values from config

Improve installation instructions in README

Add conditional to account for different OpenGL header path on macOS.

If DARWIN is true OPENGL and GLUT variables contain links against the appropriate frameworks for macOS.

Conditional name DARWIN checks if you are compiling on a macOS/OSX system.

Smaller modification of pull request #24 - compiling issues on OSX

Change needed for compilation with gcc-6

On OS X the file system usually is case-insensitive, hence installing this script will overide the executable `music`.

-Wl,-z,defs is not supported on OS X

defs
Disallows undefined symbols in object files. Undefined symbols in shared libraries are still allowed.

removing it will not hamper performance of music