[cmake] installation of communication.pdf broken

The merge of !109 (merged) broke the installation of communication.pdf:

CMake Error at doc/comm/cmake_install.cmake:36 (file):
  file INSTALL cannot find
  "/home/data/gruber/src/dune-git/dune-common/doc/comm/communication.pdf".
Call Stack (most recent call first):
  doc/cmake_install.cmake:43 (include)
  cmake_install.cmake:62 (include)

This seems to be caused by two things:

dune_add_latex_document now calls add_latex_document with the EXCLUDE_FROM_ALL option, which causes communication.pdf not to appear in the dependencies of the install target.
install seems to search for communication.pdf in the source dir instead of the build dir.