This macro isn't defined in Qt5 anymore, so use Q_OS_* where
appropriate.

Regarding bug QTCREATORBUG-7936:
Now the fonts look normal again on GNU/Linux, e.g. in the text editor
and the application/compile output pane.

Task-number: QTCREATORBUG-7936
Change-Id: I501cca608f273789d095e1138b32c2c8c38b0059
Reviewed-by: Eike Ziller <eike.ziller@digia.com>