Run EmptyLicensesLicx build target when licenses.licx file changed

[augustoproiete]

Currently we run the EmptyLicensesLicx build target just before it gets compiled. If a manual change is made to the licenses.licx file and nothing else changes in the project, a regular "Build" / "F5" won't execute our build target... The user needs to run a "Rebuild" for it to run.

We should add a dependency to the licenses.licx file so that it gets emptied on a regular build when it is the only thing that changed.