Fixes ArcticaProject/nx-libs#793.

Older autoconf versions - as used on SLE 11 and RHEL 6 - choke if
aclocal is passed an include directory that does not exist.

Instead of removing the option from the aclocal call, let's keep it and
just use an empty directory for now. Maybe we'll actually use it at a
later time.

Agent PID 0 can be used for testing

By adding NX_CLIENT=<path to nxdialog> to the envrionment
certain actions in nx will open dialogs. If the options string contains
"menu=1" rootless windows will show a popup menu when the mouse hits
the center of the title bar offering session disconnect/terminatation.