relative RPATH folders

I think might be useful to define a new variable (e.g. QMAKE_RPATHRELATIVEDIR) in order to use the token $ORIGIN in rpath folders.

This variable should be expanded as follows:

QMAKE_LFLAGS += $$join(QMAKE_RPATHRELATIVEDIR, "\',-rpath,\'\$\$ORIGIN/", "-Wl,-z,origin,-rpath,\'\$\$ORIGIN/", "\'")