[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:

1. 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.
2. install seems to search for communication.pdf in the source dir instead of the build dir.