Compatibility fixes for Mono 2.10

The Mono 2.10 compiler fails when compiling extern methods that are not
marked as DllImport. We fix that by adding a method body that throws a
NotImplementedException instead.

Additionally, MonoDevelop 2.8 cannot open sln files with ToolsVersion 12.
The fix is to change ToolsVersion to 11.

OpenTK.sln

@@ -1,6 +1,6 @@
 
-Microsoft Visual Studio Solution File, Format Version 12.00
+Microsoft Visual Studio Solution File, Format Version 11.00
-# Visual Studio 2013
+# Visual Studio 2012
 VisualStudioVersion = 12.0.21005.1
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OpenTK", "Source\OpenTK\OpenTK.csproj", "{A37A7E14-0000-0000-0000-000000000000}"

Source/Bind/CSharpSpecWriter.cs

@@ -259,7 +259,7 @@ namespace Bind
 
             sw.WriteLine("[AutoGenerated(Category = \"{0}\", Version = \"{1}\", EntryPoint = \"{2}\")]",
                 f.Category, f.Version, Settings.FunctionPrefix + f.WrappedDelegate.EntryPoint);
-            sw.WriteLine("public static extern {0};", GetDeclarationString(f, Settings.Compatibility));
+            sw.WriteLine("public static {0} {{ throw new NotImplementedException(); }}", GetDeclarationString(f, Settings.Compatibility));
         }
 
         DocProcessor processor_;