Minor CMake Update

[qnzhou]

Summary:

To make geometry-central more friendly as a dependency, the following cmake changes are added:

• Avoid building nanort, nanoflann and happly if the correspoinding cmake targets already exist.
• Move nanoflann.h out of the nanoflann directory as this is case in the original nanoflann repo. This makes geometry-central compatible with repos which interface with nanoflann directly.

[nmwsharp]

Thanks for submitting! These all look good to me.

One minor request: I think the nanoflann/nanoflann.hpp --> nanoflann.hpp change could be a small breaking change for projects that included the file from the old location. It's silly, but does it sound reasonable to add a symlink at the old location just so there isn't a needless breaking change?

[qnzhou]

does it sound reasonable to add a symlink at the old location just so there isn't a needless breaking change?

Yes, I have updated the cmake to make a symlink so #include <nanoflann/nanoflann.hpp> will still work. :)

[nmwsharp]

thank you!! merged