FLAN_DIR is wrong

[yurivict]

When external flann is used (-DFLANN_INCLUDE_DIR_HINTS=/usr/local/include), FindFlann.cmake sets FLANN_DIR=/usr/local/include/flann, to directory where flann.hpp is, although it is always included as #include <flann/flann.hpp>.

Also, it doesn't make sense that when user supplies the correct FLANN_INCLUDE_DIR_HINTS=/usr/local/include, the build still uses the internally computed FLANN_DIR. It should just use FLANN_INCLUDE_DIR_HINTS.

Another, a better way to handle this is to change the cmake variable to USE_EXTERNAL_FLANN=ON, and then to find it with pkg-config --cflags flann.