support -rpath for modules which are not installed to QT_INSTALL_PREFIX

support -rpath for modules which are not installed to QT_INSTALL_PREFIX

a module's project file may set MODULE_INSTALL_LIBS before loading
qt_module.prf to have an alternative RPATH linked into the users of that
module.
this is relevant only for linking against non-installed -prefix builds
of that module, as otherwise .libs from the module's pri file is used
for rpath.

Change-Id: Ib240e748cf130a71a5991dc643c368a983092ead
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>