include (FindDoxygen)

set_directory_properties (PROPERTIES CLEAN_NO_CUSTOM true)
set_directory_properties (PROPERTIES ADDITIONAL_MAKE_CLEAN_FILES ".built-html")

if (NOT DOXYGEN_EXECUTABLE)
    message (STATUS "WARNING: Doxygen is required to build the docs.")
else()
    add_custom_target (doc COMMENT "Building documentation..."
    DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile ${CMAKE_CURRENT_BINARY_DIR}/.built-html)
endif()

add_custom_command (OUTPUT .built-html
    COMMAND sh
    ARGS -c \"${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile && touch ${CMAKE_CURRENT_BINARY_DIR}/.built-html\;\"
    ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
)