enable ExtensionAttribute polyfills

Author: sajagi

src/Serilog.Settings.Configuration/Settings/Configuration/ConfigurationReader.cs

@@ -511,12 +511,16 @@ static IList<MethodInfo> FindEventEnricherConfigurationMethods(IReadOnlyCollecti
 
         static List<MethodInfo> FindConfigurationExtensionMethods(IReadOnlyCollection<Assembly> configurationAssemblies, Type configType)
         {
+            // ExtensionAttribute can be polyfilled to support extension methods
+            bool hasExtensionAttribute(MethodInfo m) =>
+                m.CustomAttributes.Any(a => a.AttributeType.FullName == "System.Runtime.CompilerServices.ExtensionAttribute");
+
             return configurationAssemblies
                 .SelectMany(a => a.ExportedTypes
                     .Select(t => t.GetTypeInfo())
                     .Where(t => t.IsSealed && t.IsAbstract && !t.IsNested))
                 .SelectMany(t => t.DeclaredMethods)
-                .Where(m => m.IsStatic && m.IsPublic && m.IsDefined(typeof(ExtensionAttribute), false))
+                .Where(m => m.IsStatic && m.IsPublic && hasExtensionAttribute(m))
                 .Where(m => m.GetParameters()[0].ParameterType == configType)
                 .ToList();
         }