don't set the -rpath flag to the install dir as this breaks DESTDIR

--- a/SConstruct
+++ b/SConstruct
@@ -287,7 +287,7 @@
 
 # Don't hack RPATH unless libdir points somewhere that is not on the
 # minimum default load path.
-if env["shared"]:
+if env["shared"] and env["chrpath"]:
     if env["libdir"] not in ["/usr/lib", "/lib"]:
         env.Prepend(RPATH=[installdir('libdir')])