Fix building protobufgen with old protobuf versions

Guard the 'override' keyword of GetSupportedFeatures with protobuf
version check that is only applicable for versions higher than 3.12.

TODO: This workaround should gone once we restrict the minimal supported
protobuf version. See QTBUG-115702.

Amends bbb19e18e2

Fixes: QTBUG-115693
Pick-to: 6.6
Change-Id: I9cba91215b8914d5994bfa27114a7deb411f8756
Reviewed-by: Konrad Kujawa <konrad.kujawa@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/tools/qtprotoccommon/generatorbase.cpp

@@ -29,11 +29,6 @@ bool GeneratorBase::GenerateAll(const std::vector<const FileDescriptor *> &files
     return CodeGenerator::GenerateAll(files, parameter, generatorContext, error);
 }
 
-uint64_t GeneratorBase::GetSupportedFeatures() const
-{
-    return CodeGenerator::Feature::FEATURE_PROTO3_OPTIONAL;
-}
-
 std::string GeneratorBase::generateBaseName(const FileDescriptor *file, const std::string &name)
 {
     std::string outFileBasename;

src/tools/qtprotoccommon/generatorbase.h

@@ -6,6 +6,7 @@
 #define GENERATORBASE_H
 
 #include <google/protobuf/compiler/code_generator.h>
+#include <google/protobuf/port_def.inc>
 
 #include <string>
 #include <functional>
@@ -37,7 +38,17 @@ public:
                      std::string *error) const override;
     bool HasGenerateAll() const override { return true; }
 
-    uint64_t GetSupportedFeatures() const override;
+// TODO: This suppresses the build issue with old protobuf versions. Since we don't have the
+// strict protobuf versions that we support this work around will be here for a while, since
+// yocto builds quite old protobuf by now. See QTBUG-115702.
+#if PROTOBUF_VERSION < 3012000
+    uint64_t GetSupportedFeatures() const { return 0; };
+#else
+    uint64_t GetSupportedFeatures() const override
+    {
+        return CodeGenerator::Feature::FEATURE_PROTO3_OPTIONAL;
+    }
+#endif
 
     static void printDisclaimer(::google::protobuf::io::Printer *printer);
     void OpenFileNamespaces(const ::google::protobuf::FileDescriptor *file,
@@ -51,4 +62,6 @@ protected:
 };
 } // namespace qtprotoccommon
 
+#include <google/protobuf/port_undef.inc>
+
 #endif // GENERATORBASE_H