When debugging a CMake build file it is often useful to print out all the directories included by a target. The following snippet does just that. Simply replace ${target_name} and run cmake to see the list of included files.

get_property(includes
        TARGET ${target_name}
        PROPERTY INCLUDE_DIRECTORIES
        )

message(STATUS "Included directories for target ${target_name}")
for (dir in ${includes})
    message(STATUS "  - ${dir}")
endfor()