protobuf 3.18.0 broken for Windows when building with MinGW-w64

[brechtsanders]

protobuf 3.18.0 fails to build on Windows with MinGW-w64 GCC with the following errors:

R:/winlibs64-10.3.0/protobuf-3.18.0/src/google/protobuf/io/zero_copy_stream_impl.cc:108:28: error: 'F_GETFL' was not declared in this scope
  108 |   int flags = fcntl(file_, F_GETFL);
      |                            ^~~~~~~
R:/winlibs64-10.3.0/protobuf-3.18.0/src/google/protobuf/io/zero_copy_stream_impl.cc:108:15: error: 'fcntl' was not declared in this scope
  108 |   int flags = fcntl(file_, F_GETFL);
      |               ^~~~~
R:/winlibs64-10.3.0/protobuf-3.18.0/src/google/protobuf/io/zero_copy_stream_impl.cc:109:13: error: 'O_NONBLOCK' was not declared in this scope
  109 |   flags &= ~O_NONBLOCK;
      |             ^~~~~~~~~~
R:/winlibs64-10.3.0/protobuf-3.18.0/src/google/protobuf/io/zero_copy_stream_impl.cc:110:16: error: 'F_SETFL' was not declared in this scope
  110 |   fcntl(file_, F_SETFL, flags);
      |                ^~~~~~~

The solution is to replace _MSC_VER with _WIN32 in src/google/protobuf/io/zero_copy_stream_impl.cc.

Apparently a false assumption was made _MSC_VER is always set on the Windows platform, but that is only the case when building with MSVC. On MSVC and MinGW gcc and clang _WIN32 is always defined however.

[jinmingyi1998]

Yes! only this version encounter this error