Missing external dependencies needed for building the targets in `doc/` can cause the build to fail

Commenting out

ADD_SUBDIRECTORY(doc)

allows me to successfully configure the build.

logfile:

CMake Error at cmake/LATEX.cmake:1052 (message):
  Could not find convert program.  Please download ImageMagick from
  http://www.imagemagick.org and install.
Call Stack (most recent call first):
  cmake/LATEX.cmake:1119 (latex_add_convert_command)
  cmake/LATEX.cmake:1154 (latex_convert_image)
  cmake/LATEX.cmake:1401 (latex_process_images)
  cmake/LATEX.cmake:1791 (add_latex_targets_internal)
  doc/CMakeLists.txt:101 (ADD_LATEX_DOCUMENT)