Only set the deprecated attribute if hidden visibilty is enabled.

This is what is tested in the unit test. If it makes sense, another
option can be added later.

Modules/GenerateExportHeader.cmake

@@ -155,7 +155,9 @@ macro(_DO_SET_MACRO_VALUES TARGET_LIBRARY)
       set(DEFINE_DEPRECATED "__declspec(deprecated)")
     endif()
   else()
-    set(DEFINE_DEPRECATED "__attribute__ ((__deprecated__))")
+    if(COMPILER_HAS_HIDDEN_VISIBILITY AND USE_COMPILER_HIDDEN_VISIBILITY)
+      set(DEFINE_DEPRECATED "__attribute__ ((__deprecated__))")
+    endif()
   endif()
 
   get_property(type TARGET ${TARGET_LIBRARY} PROPERTY TYPE)