i#1950: Fix some Windows builds.
i#1950: Fix some Windows builds.

Avoid empty list of target dependencies in add_dependencies, which gives
the error "add_dependencies called with incorrect number of arguments"
with some tool chains on some systems.

Fixes a regression from d966e7d.

Fixes #1950

Review-URL: https://codereview.appspot.com/298480043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio