We always build PolicyKit support, because it merely depends on some
D-Bus calls. However, there are two things to configure:
- the default value for main.auth-polkit in NetworkManager.conf. This
is now called "-Dconfig_auth_polkit_default=$VAL".
- whether to install the policy file. This is called "-Dpolkit=$VAL".
These settings are mostly independent, so add "config_auth_polkit_default" to
make the default explicitly configurable.