[C++] warnings building on mingw-w64

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Two warnings when building libarrow 0.15.1 on mingw-w64:

[  2%] Running thrift compiler on parquet.thrift
[WARNING:C:/msys64/home/mingw-packages/mingw-w64-arrow/src/apache-arrow-0.15.1/cpp/src/parquet/parquet.thrift:297] The "byte" type is a compatibility alias for "i8". Use "i8" to emphasize the signedness of this type.

And later:

 81%] Building CXX object src/parquet/CMakeFiles/parquet_static.dir/column_reader.cc.obj
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/apache-arrow-0.15.1/cpp/src/parquet/arrow/writer.cc: In member function 'virtual arrow::Status parquet::arrow::FileWriterImpl::WriteColumnChunk(const std::shared_ptr<arrow::ChunkedArray>&, int64_t, int64_t)':
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/apache-arrow-0.15.1/cpp/src/parquet/arrow/writer.cc:79:41: warning: 'schema_field' may be used uninitialized in this function [-Wmaybe-uninitialized]
         schema_manifest_(schema_manifest) {}
                                         ^
C:/msys64/home/mingw-packages/mingw-w64-arrow/src/apache-arrow-0.15.1/cpp/src/parquet/arrow/writer.cc:466:24: note: 'schema_field' was declared here
     const SchemaField* schema_field;

Maybe CI with CXXFLAGS += -Werror ?

Reporter: Jeroen / @jeroen
Assignee: Ben Kietzman / @bkietz

PRs and other links:

• GitHub Pull Request #6691

_{Note: This issue was originally created as ARROW-7049. Please see the migration documentation for further details.}

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Antoine Pitrou / @pitrou:
@jeroen Do you want to provide a PR for this?

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Francois Saint-Jacques / @fsaintjacques:
These warnings are fixed on master. @mrkn can you confirm that there is no other warnings?

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Neal Richardson / @nealrichardson:
There are a few others now, see the build log at https://github.com/apache/arrow/runs/527579208?check_suite_focus=true#step:7:1096 for example

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Ben Kietzman / @bkietz:
Some of those warnings are spurious and arise from generated code, so -Werror isn't viable here. The warnings regarding FieldRef are semi legitimate, I'll open a PR to fix them

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Ben Kietzman / @bkietz:
Issue resolved by pull request 6691
#6691