installpanda: add note about needing to call "sudo ldconfig"

This note only appears when not using --destdir

This should help avoid user confusion (such as in #818)

[skip ci]

makepanda/installpanda.py

@@ -322,3 +322,8 @@ if (__name__ == "__main__"):
                      outputdir=options.outputdir,
                      python_versions=ReadPythonVersionInfoFile())
     print("Installation finished!")
+
+    if not destdir:
+        warn_prefix = "%sNote:%s " % (GetColor("red"), GetColor())
+        print(warn_prefix + "You may need to call this command to update the library cache:")
+        print("  sudo ldconfig")