Doc: Change instances of 'OS X' to 'macOS'

As of version 10.12 (Sierra), the name of Apple's desktop operating
system will be macOS. Change all occurrences where the Mac platform
is discussed to use the macro \macos (defined in the documentation
configuration in qtbase).

Change-Id: I0a7da987b0322b622a34f6735210bfdf708479ab
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>