Build should have an option to use system dependencies instead of bundling anything

[dkogan]

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Hi. I just pushed Alicevision into Debian. A number of patches were required to make the build work against the system libraries. It would be great if some of these could be merged, at least with a build flag. The patches:

• Using the system COIN solvers: alicevision has its own patched flavor of COIN, which provides its own cmake rules. If these are really needed, can they be contributed to the upstream coin project? In any case, you don't need cmake rules to "find" system libraries: they're in standard locations, and can be used directly, as in this patch
• Using the system MeshSDFilter
• Using the system nanoflann
• Refer to the "lemon" library with a working name: Debian's copy of lemon ships lemonConfig.cmake, not LEMONConfig.cmake
• Optimizing for the build system should be optional because we might want to run on a different machine than the build machine. We might even want to cross-build

There are other patches that address more specific issues that I will report as separate bugs. Thanks for Alicevision!