Support system-wide settings.

Change-Id: I31a5cc8d68696ac2d1c4a02b5340854d1d7ee060
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
fix setup-qt for Qt 5.2

The location of the mkspec is in the source dir for developer builds.

Change-Id: I3374763fae041568b989865dbad24fd6c7932e02
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
store settings on OS X within a group

QSettings::allKeys returns extra keys on OS X we're not interested in.
We worked around this "feature" by setting QSettings::fallbacksEnabled
to false which is now needed to support a system-wide configuration.

Old settings are migrated, i.e. moved to the internal group.

Change-Id: I59f6672192f2e92bf6e5cddc679374b7b25965d5
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>