message: make 'mkdoc' target optional
diff: @@ -313,7 +313,7 @@ if (Sphinx_FOUND)
  set(SPHINX_INPUT_DIR "${CMAKE_CURRENT_SOURCE_DIR}/docs")
  set(SPHINX_OUTPUT_DIR "${CMAKE_CURRENT_BINARY_DIR}/html")
  - add_custom_target(mkdoc ALL
  + add_custom_target(mkdoc
  ${SPHINX_EXECUTABLE} -b html "${SPHINX_INPUT_DIR}" "${SPHINX_OUTPUT_DIR}"
  COMMENT "Building HTML documentation with Sphinx"
  USES_TERMINAL)
  