Bumping FLANN to 1.8.2 and adding OpenMP support

FLANN got updated to 1.8.2 on 20 December 2012.
This bumps up the homebrew version accordingly.

Additionally, I found that to have OpenMP support (for parallel search), the only change that needs to happen is to make sure that cmake uses `g++` and `gcc` instead of `c++` and `cc`.

This probably fixes #16317, as I am also 10.8.2 and it works.