This tests appropriate behavior when a project (or multi-project) tries to
create multiple documents with multiple add_latex_document calls. It also
creates them in separate subdirectories, with each subdirectory
independently loading UseLATEX.cmake. This is typical in a large project
that might bring in several subprojects.
When running this project, two documents should be built. You should not
get errors or warnings about a target being defined multiple times. You
should be able to build using the pdf, dvi, ps, and safepdf targets.