Fix the build for GCC

src/openrct2/ParkFile.cpp

@@ -866,7 +866,10 @@ int32_t scenario_save(const utf8* path, int32_t flags)
 class ParkFileImporter : public IParkImporter
 {
 private:
-    [[maybe_unused]] const IObjectRepository& _objectRepository;
+#ifdef __clang__
+    [[maybe_unused]]
+#endif
+    const IObjectRepository& _objectRepository;
     std::unique_ptr<OpenRCT2::ParkFile> _parkFile;
 
 public:

src/openrct2/core/OrcaStream.hpp

@@ -253,9 +253,14 @@ namespace OpenRCT2
                 return v;
             }
 
+#if defined(_MSC_VER) || defined(__clang__)
             template<> void ReadWrite(std::string_view& v) = delete;
+#endif
 
-            template<> void ReadWrite(std::string& v)
+#if defined(_MSC_VER) || defined(__clang__)
+            template<>
+#endif
+            void ReadWrite(std::string& v)
             {
                 if (_mode == Mode::READING)
                 {
@@ -280,7 +285,10 @@ namespace OpenRCT2
                 }
             }
 
-            template<> void Write(const std::string_view& v)
+#if defined(_MSC_VER) || defined(__clang__)
+            template<>
+#endif
+            void Write(const std::string_view& v)
             {
                 if (_mode == Mode::READING)
                 {

src/openrct2/rct2/S6Exporter.cpp

@@ -1815,7 +1815,7 @@ enum : uint32_t
  *  rct2: 0x006754F5
  * @param flags bit 0: pack objects, 1: save as scenario
  */
-int32_t scenario_save_old(const utf8* path, int32_t flags)
+[[maybe_unused]] static int32_t scenario_save_old(const utf8* path, int32_t flags)
 {
     if (flags & S6_SAVE_FLAG_SCENARIO)
     {