Restore protobuf-2.x compatibility

The change in commit 712154b9 ("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.

protoc-c/c_file.cc

@@ -120,9 +120,11 @@ void FileGenerator::GenerateHeader(io::Printer* printer) {
   string filename_identifier = FilenameIdentifier(file_->name());
 
   int min_header_version = 1000000;
+#if defined(HAVE_PROTO3)
   if (file_->syntax() == FileDescriptor::SYNTAX_PROTO3) {
     min_header_version = 1003000;
   }
+#endif
 
   // Generate top of header.
   printer->Print(