Adds flycheck-clang-include-path support.

Parses cppcm-include-dirs to set flycheck-clang-include-path.

Also pulls "add_library" and "add_executable" mentions into a user-customizable variable to allow for cases when the user has changed the default CMake syntax (e.g. "def_library" from https://github.com/arpg/rpg-cmake to clean up my CMake files).

I'd like to add more flycheck-clang support in the future, as well.