The change in commit 712154b912de824741381c0bb26c2fbed54515a3 ("Bump
minimum required header version for proto3 syntax") uses functionality
only exposed by protobuf-3.x, breaking the build when compiling against
protobuf-2.x.
Since we still want to support building against protobuf-2.x, this
commit makes the proto3 syntax check in the file generator dependent on
building against protobuf-3.x.
4302266b45