Improve backward compatibility of macOS code

[barracuda156]

Is your feature request related to a problem? Please describe.
Existing code cannot be compiled on earlier versions of macOS or on any version of macOS with gcc.

Describe the solution you'd like
Provide fallbacks in macos.mm, so that modern macOS systems use the bleeding edge code (i.e. nothing changes for them), while legacy systems fall back to compatible syntax.

Describe alternatives you've considered
Alternative would be to use compatible syntax regardless of macOS version, but it is a) perhaps suboptimal and b) nobody will be interested to make radical changes for the sake of a small pool of users.

Additional context
From what I see, the changes are minimal, and I get a working binary on 10.6.8 on PowerPC. If such PR will be accepted, I can make that.

[barracuda156]