Add pg_config libdir to the library's rpath

Close #1356.

Author: Giorgos Tzampanakis

setup.py

@@ -372,7 +372,9 @@ def finalize_options(self):
             self.libraries.append("pq")
 
         try:
-            self.library_dirs.append(pg_config_helper.query("libdir"))
+            pg_config_libdir = pg_config_helper.query("libdir")
+            self.library_dirs.append(pg_config_libdir)
+            self.rpath.append(pg_config_libdir)
             self.include_dirs.append(pg_config_helper.query("includedir"))
             self.include_dirs.append(pg_config_helper.query("includedir-server"))