Fall back to exo-csource if xdt-csource isn't found

Some time ago, the exo-csource tool had been move out from exo into xfce4-dev-tools and renamed to xdt-csource. The drawback is an unnecessary version lock-in between several packages, just because the name has changed.

In order to allow upgrading xfdesktop individually (eg. in regulated environments where each upgrade causes a massive paper trail), just make a trivial change to support both, old and new command name.

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net