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ARROW-3470: [C++] Fix row-wise example

- Implement the `ADD_EXAMPLE` cmake function with new ctest label
  `example`, also covered by the `runexample` target. This can be
  toggled via the `ARROW_BUILD_EXAMPLES` option which is ON by default.
- Implement fully working `row-wise-conversion-example.cc` and add it to
  the default build.
- Update documentation to embed (manually) the newly created example.

Author: François Saint-Jacques <fsaintjacques@gmail.com>

Closes #3078 from fsaintjacques/ARROW-3470-out-of-date-example and squashes the following commits:

fab63f6f <François Saint-Jacques> ARROW-3470: Fix status macro
1eba067d <François Saint-Jacques> ARROW-3470:  Fix row-wise example