Repurpose the former Q_OS_MAC synonym "Q_OS_MACX" to mean "OS X".

The equivalent was done earlier for qmake, and this will enable
developers writing OS X-specific code to use #ifdef Q_OS_MACX as
opposed to overly verbose #if defined(Q_OS_MAC) && !defined(Q_OS_IOS).

The sole usage of Q_OS_MACX within qtbase has been changed to the
now appropriate value and documentation has been updated.

Change-Id: I2c59eea02e94b691b705170b2f96a97940fdc756
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>