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ARROW-1289: [Python] Add PYARROW_BUILD_PLASMA CMake option, follow semantics of --with-parquet

Now the test suite does not fail if you build the Plasma libraries but forget to pass `--with-plasma` to the Python build

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

Closes #903 from wesm/ARROW-1289 and squashes the following commits:

0e9ce78b [Wes McKinney] Add PYARROW_BUILD_PLASMA CMake option, make plasma build work like parquet build