sound: add a soundlib interface that consists of a specification
documented in doc/sound.txt; default 'nosound' soundlib implementation
of the specification in src/sounds.c
-sound: added soundlib_macsound implementation for use on macOS; the
+sound: added 'macsound' soundlib implementation for use on macOS; the
interface implementation resides in the objective C file
sound/macsound/macsound.m; built on top of the built-in AppKit
NSSound, #import <AppKit/AppKit.h>, linked using -framework AppKit.