Make -no-rpath build more useful on Apple platforms

[ChangeLog][Platform Specific Changes][OS X] Configure with -no-rpath
will now yield Qt dynamic libraries and frameworks with an absolute
install name (based in -libdir).

OS X package managers like Homebrew install Qt in a fixed location. This
change simplifies deployment for such package managers and is consistent
with the default expectation on Apple platforms for libraries with a
fixed location to also have absolute install names.

While a relocatable installation (the default) also works in this
scenario, it requires all software that depends on Qt to be aware of
this and to embed a suitable RPATH into application binaries (which is
not automatic for non-qmake builds). This might not be true for some
select fallback search locations, but as package managers on OS X tend
not to use those, embedding an RPATH becomes practically mandatory. In a
default Homebrew installation, Qt is configured such that the frameworks
end up in /usr/local/Cellar/qt5/<version>/lib and that will be later
symlinked to /usr/local/opt/qt5/lib, both of which are not searched by
the dynamic linker by default.

Task-number: QTBUG-48958
Change-Id: I4395df98771e06a2ce8a293d11dc755bdc50757f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>