Build: make compatibility backport attributes internal only

MathNet.Numerics.All.sln

@@ -45,6 +45,7 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "UnitTests-Net35", "src\Unit
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Build", "Build", "{B54A0B40-DE22-49FB-B1C0-6E5BDA3B0B2F}"
 	ProjectSection(SolutionItems) = preProject
+		build\build-framework.fsx = build\build-framework.fsx
 		build.fsx = build.fsx
 		build\MathNet.Numerics.nuspec = build\MathNet.Numerics.nuspec
 		paket.dependencies = paket.dependencies

src/Numerics/Compatibility.cs

@@ -8,12 +8,12 @@ namespace MathNet.Numerics
     using System.Threading.Tasks;
 
     [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
-    public class SerializableAttribute : Attribute
+    internal class SerializableAttribute : Attribute
     {
     }
 
     [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
-    public class SpecialNameAttribute : Attribute
+    internal class SpecialNameAttribute : Attribute
     {
     }
 
@@ -145,7 +145,7 @@ namespace MathNet.Numerics
     using System;
 
     [AttributeUsage(AttributeTargets.Constructor | AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
-    public class TargetedPatchingOptOutAttribute : Attribute
+    internal class TargetedPatchingOptOutAttribute : Attribute
     {
         public string Reason { get; private set; }