Included version of Flann doesn't build with CMake 3.11

[predicative]

The included version of Flann is affected by flann-lib/flann#369 (still open), where it won't build with CMake 3.11 or later. The linked issue has a potential work-around.

[Purg]

Valid. I have been avoiding this issue by disabling the FLANN build since it hasn't been recently used in favor of the the LSH plugin. Do you have a specific use case currently that necessitates the use of FLANN?

[predicative]

My use case was building VIAME, but it appears we don't really need it there either and we've disabled it. Feel free to mark this as low-priority.