cmake: fix Ninja warning

about BYPRODUCTS bin/opencv-340.jar

modules/java/jar/CMakeLists.txt

@@ -21,7 +21,9 @@ set(depends ${the_module}_jar_source_copy "${OPENCV_DEPHELPER}/${the_module}_jar
 configure_file("${CMAKE_CURRENT_SOURCE_DIR}/build.xml.in" "${OPENCV_JAVA_DIR}/build.xml" @ONLY)
 list(APPEND depends "${OPENCV_JAVA_DIR}/build.xml")
 
+ocv_cmake_byproducts(__byproducts BYPRODUCTS "${OPENCV_JAR_FILE}")
 add_custom_command(OUTPUT "${OPENCV_DEPHELPER}/${the_module}_jar"
+    ${__byproducts}  # required for add_custom_target() by ninja
     COMMAND ${ANT_EXECUTABLE} -noinput -k jar
     COMMAND ${CMAKE_COMMAND} -E touch "${OPENCV_DEPHELPER}/${the_module}_jar"
     WORKING_DIRECTORY "${OPENCV_JAVA_DIR}"