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>
Build Qt for OS X and iOS with relative rpath

Defaulting to absolute_library_soname on configure -rpath is no longer
necessary as now we support @rpath install name ids on OS X and iOS.

This also sets QMAKE_SONAME_PREFIX to @rpath for Qt modules when built with
rpath configuration.

This makes Qt libraries relocatable on OS X. Qt SDK is not yet relocatable
though, because plugin location (including cocoa plugin) is still resolved
using absolute path (see QTBUG-14150), also there are several absolute paths
hardcoded in qmake mkspecs pri files.

Task-number: QTBUG-31814
Change-Id: Ie9dffefcd2a946c1580293d433621c1adb7e06c4
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>