Silence GCC's extra warnings

GCC warned about ignored attributes that we use for clang and MSVC.
Removing the ignored attributes check keeps this quiet

CMakeLists.txt

@@ -368,6 +368,11 @@ if( CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang" )
         set( CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fstandalone-debug" )
         set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fstandalone-debug" )
     endif()
+    
+    # Suppress GCC warnings about unknown/unused attributes (e.g. cdecl, [[maybe_unused, etc)
+    if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+        set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-attributes" )
+    endif()
 
     if( KICAD_SANITIZE_ADDRESS )
         add_definitions( -DKICAD_SANITIZE_ADDRESS )

include/lib_tree_model_adapter.h

@@ -393,12 +393,7 @@ protected:
     LIB_TREE_NODE_ROOT      m_tree;
 
 private:
-    #ifndef __clang__
-    // [[maybe_unused]] attribute is ignored by Gcc but generates a warning.
-    EDA_BASE_FRAME*         m_parent;
-    #else
     [[maybe_unused]] EDA_BASE_FRAME* m_parent;
-    #endif
 
     SYM_FILTER_TYPE         m_filter;
     bool                    m_show_units;