Make sure to use defined(__GNUC__) instead of bare __GNUC__ in flann2 header.
Make sure to use defined(__GNUC__) instead of bare __GNUC__ in flann2 header.

This fixes a compiler error on MSVC with Unreal Engine (at least).

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv