There are countless threads describing what is RPATH, RUNPATH and LD_LIBRARY_PATH and how they interact in order to resolve libraries\
RPATH
RUNPATH
LD_LIBRARY_PATH
Seems runpath is set to the -Wl value if you pass --enable-new-dtags
http://blog.tremily.us/posts/rpath/
This links seems to suggest the difference in use for rpath and runpath seems to be their order in the search list:
http://en.wikipedia.org/wiki/Rpath