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ARROW-285: Optional flatc download

For platforms which don't have a flatc compiler artifact on maven central,
allow to skip the download and manually provide a flatc compiler

usage: mvn -Dflatc.download.skip -Dflatc.executable=/usr/local/bin/flatc

Author: Laurent Goujon <laurent@dremio.com>

Closes #129 from laurentgo/laurent/optional-flatc-download and squashes the following commits:

229c6d5 [Laurent Goujon] Optional flatc download