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ARROW-616: [C++] Do not include debug symbols in release builds by default

This reduces binary size on Linux by about 80-90%. If the user wants them, they can enable with `-DARROW_CXXFLAGS="-g"`.

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #574 from wesm/ARROW-616 and squashes the following commits:

71fc105 [Wes McKinney] Do not include debug symbols in release builds by default