Turn on HarfBuzz support for Mac/Cocoa

Turn on HarfBuzz support for Mac/Cocoa

It's possible to enable HarfBuzz text layout on Mac by either:

- Set QT_ENABLE_HARFBUZZ environment variable when running a Qt
  app.

- configure with -harfbuzz to build Qt, then HarfBuzz support
  will be turned on by default.

HarfBuzz will only be used when the font explicitly requested
is supported by HarfBuzz (aka. TrueType/OpenType font), other
fonts (AAT fonts) will still be handled by Core Text.

Using HarfBuzz for text layout will hopefully solve most tricky
complex text shaping bugs on Mac.

Task-number: QTBUG-17728
Reviewed-by: Eskil