Support for CM108-based PTT on Mac is provided using the hidapi library
in the same way as on Windows. As such, the code changes are limited
almost entirely to updated #if conditions, treating Windows and Mac in
the same way.

The CMake changes are slightly complicated by the Windows build using
a local copy of some hidapi files, for some reason, instead of using
the hidapi library itself. The Mac version uses hidapi in the same way
as other libraries.

In the CMake files, it is unclear to me whether "elseif (NOT WIN32 AND
NOT CYGWIN)" means the same thing as "elseif (APPLE)", so they are
treated separately in order to avoid breaking other build types.

The generic config file can now be simplified slightly, since the
section on using CM108 for PTT is now common to all of Linux, Windows
and Mac.

Since building on Mac using Homebrew is straightforward, include this
information in a new section of the README.