As of version 10.12 (Sierra), the name of Apple's desktop operating
system will be macOS. Change the occurrences where the Mac platform
is discussed to use a macro \macos, which expands to 'macOS'. This
helps with adapting to future renaming.
Update the instructions on mac-specific Q_OS_* macro usage.
Add a \target for the old 'Qt for OS X' topic to keep links working
for other documentation modules that try to link with the old name.
Change-Id: Id33fb0cd985df702a4ae4efb4c5fd428e77d9b85
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>