Make zlib/jpeg/png/... auto link to native libraries on Ubuntu/OSX

[qianyizh]

On Windows, all packages are included in External, and linked with Open3D.
On Ubuntu/OSX, this should be done automatically with native libraries.
This should be done with findpackage in CMake.

Roadblock: Need to do it and thoroughly test if these dependencies (a weird version, e.g.) break Open3D. In that case, we should fall back to the code included in External.

[qianyizh]

Starting to work on this issue from e6ae72c

[qianyizh]

A series of commits have addressed the issue.
From e6ae72c to 861c5de, the following libraries have two configurations:

• Eigen
• Glew
• Glfw
• libjpeg
• libpng
• jsoncpp

A few libraries that have not changed:

• flann (header only, old and stable)
• liblzf (old and stable)
• rply (old and stable)
• tinyfiledialog (experimental)
• pybind11 (experimental)
• librealsense (should be auto-configured, did not get time to do it)

Moving the priority to low. I will revisit it in the future when librealsense is more stable.

[qianyizh]

Close for now.