Add rpath pointing to Qt libraries in OS X and iOS

This is triggered only when app is using Qt and Qt was built with "rpath"
configuration and project does not specify QMAKE_RPATHDIR explicitly.

Added rpath is made relative to app binary location if target path lies inside
Qt SDK, so all SDK bundled tools and examples will work automatically without
any changes. Tests are an exception here, since they are being run from their
build location by CI, we may not use relative rpath that work only in install
location.

Task-number: QTBUG-31814
Change-Id: I3690f29d2b5396a19c1dbc92ad05e6c028f8515b
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>