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, 3 documents should be built, one in each
subdirectory. 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.