CMake: Manually specify the path to cldoc

In the event the user invoking CMake does not have cldoc installed
on the system path, the CLDOC variable can be set manually using
cmake -DCLDOC:STRING=/path/to/cldoc /path/to/CMakeLists.txt
Merge pull request #37 from kbenzie/master

CMake: Manually specify the path to cldoc