mjpg-streamer input plugin: input_opencv
This input plugin uses OpenCV to read from supported video sources, optionally running the image through a filter plugin that can be specified on the command line.
If you're not using the image filtering functionality of this plugin, you're probably better off using some other input plugin as this plugin will probably consume more CPU resources.
This plugin has only been tested with OpenCV 3.1.0, will probably not work with OpenCV 2.x without some adjustments.
Solution:
Do you want to use the OpenCV plugin? If not you can just remove add_subdirectory(plugins/input_opencv) from CMakeLists.txt and do a make distclean before make