[C++][Parquet] Fix compiler warnings on OS X / Clang

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There have been two patches addressing OS X-related issues:

apache/parquet-cpp#15
apache/parquet-cpp#6

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as PARQUET-455. Please see the migration documentation for further details.}

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Wes McKinney / @wesm:
see apache/parquet-cpp#40

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Julien Le Dem / @julienledem:
Issue resolved by pull request 40
apache/parquet-cpp#40