Skip LATEX_USABLE guard for communication.pdf as the check is incorporated into dune_add_latex_document.

[[Imported from SVN: r7405]]

The tarballs already contain the generated pdf files. No CMake 
will detect whether the pdf are contained in the source tree.
If that is the case they will not be generated and installed directly.

[[Imported from SVN: r7380]]

[[Imported from SVN: r7116]]