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>