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

--- a/SConstruct
+++ b/SConstruct
@@ -268,7 +268,7 @@ def installdir(dir, add_destdir=True):
 
 # Honor the specified installation prefix in link paths.
 env.Prepend(LIBPATH=[installdir('libdir')])
-if env["shared"]:
+if env["shared"] and env["chrpath"]:
     env.Prepend(RPATH=[installdir('libdir')])
 
 # Give deheader a way to set compiler flags