Add BUILD_DOCUMENTATION and/or BUILD_EXAMPLE only when CMakeLists.txt found

[schuhschuh]

Currently, these options are added when the PROJECT_DOC_DIR and/or PROJECT_EXAMPLE_DIR exists even when these directories are empty or do not contain a required CMakeLists.txt file. Considering that projects may have such directories which do not require any built, add the options only when a CMakeLists.txt file is found.

[ahundt]

Perhaps it would also make sense to handle when the directories simply don't exist? Or is that already taken care of?

[schuhschuh]

Perhaps it would also make sense to handle when the directories simply don't exist? Or is that already taken care of?

That's how it has been. Now instead it requires an existence of the CMakeLists.txt file within those directories, which implicitly checks for existence of the directory ;-).