[CI][C++] Mingw32 builds failing

https://ci.appveyor.com/project/ApacheSoftwareFoundation/arrow/builds/22762420/job/4xuv4mpnwvmq5byf

 
C:/projects/arrow/cpp/src/arrow/util/compression_snappy.cc: In member function 'virtual int64_t arrow::util::SnappyCodec::MaxCompressedLen(int64_t, const uint8_t*)':
385C:/projects/arrow/cpp/src/arrow/util/compression_snappy.cc:76:47: error: conversion to 'size_t {aka unsigned int}' from 'int64_t {aka long long int}' may alter its value [-Werror=conversion]
386 return snappy::MaxCompressedLength(input_len);
387 ^
388cc1plus.exe: all warnings being treated as errors
389make[2]: *** [src/arrow/CMakeFiles/arrow_objlib.dir/build.make:89: src/arrow/CMakeFiles/arrow_objlib.dir/util/compression_snappy.cc.obj] Error 1
390make[2]: *** Waiting for unfinished jobs....
391C:/projects/arrow/cpp/src/arrow/util/compression_zstd.cc: In member function 'virtual int64_t arrow::util::ZSTDCodec::MaxCompressedLen(int64_t, const uint8_t*)':
392C:/projects/arrow/cpp/src/arrow/util/compression_zstd.cc:232:38: error: conversion to 'size_t {aka unsigned int}' from 'int64_t {aka long long int}' may alter its value [-Werror=conversion]
393 return ZSTD_compressBound(input_len);
394 ^
395cc1plus.exe: all warnings being treated as errors
396make[2]: *** [src/arrow/CMakeFiles/arrow_objlib.dir/build.make:63: src/arrow/CMakeFiles/arrow_objlib.dir/util/compression_zstd.cc.obj] Error 1
397make[1]: *** [CMakeFiles/Makefile2:1102: src/arrow/CMakeFiles/arrow_objlib.dir/all] Error 2
398make[1]: *** Waiting for unfinished jobs....
399-- googletest_ep build command succeeded. See also C:/projects/arrow/cpp/build/googletest_ep-prefix/src/googletest_ep-stamp/googletest_ep-build-*.log
400[ 13%] Performing install step for 'googletest_ep'
401-- googletest_ep install command succeeded. See also C:/projects/arrow/cpp/build/googletest_ep-prefix/src/googletest_ep-stamp/googletest_ep-install-*.log
402[ 13%] Completed 'googletest_ep'
403[ 13%] Built target googletest_ep
404make: *** [Makefile:141: all] Error 2
405
406