Allow protoc to be targeted by protogen explicitly.
Use that within the build for the address book sample.
diff --git a/src/AddressBook/AddressBookProtos.cs b/src/AddressBook/AddressBookProtos.cs
index e544e4c..38029ac 100644
--- a/src/AddressBook/AddressBookProtos.cs
+++ b/src/AddressBook/AddressBookProtos.cs
@@ -31,16 +31,13 @@
     

     static AddressBookProtos() {

       byte[] descriptorData = global::System.Convert.FromBase64String(

-          "Chp0dXRvcmlhbC9hZGRyZXNzYm9vay5wcm90bxIIdHV0b3JpYWwaJGdvb2ds" + 

-          "ZS9wcm90b2J1Zi9jc2hhcnBfb3B0aW9ucy5wcm90byLaAQoGUGVyc29uEgwK" + 

-          "BG5hbWUYASACKAkSCgoCaWQYAiACKAUSDQoFZW1haWwYAyABKAkSKwoFcGhv" + 

-          "bmUYBCADKAsyHC50dXRvcmlhbC5QZXJzb24uUGhvbmVOdW1iZXIaTQoLUGhv" + 

-          "bmVOdW1iZXISDgoGbnVtYmVyGAEgAigJEi4KBHR5cGUYAiABKA4yGi50dXRv" + 

-          "cmlhbC5QZXJzb24uUGhvbmVUeXBlOgRIT01FIisKCVBob25lVHlwZRIKCgZN" + 

-          "T0JJTEUQABIICgRIT01FEAESCAoEV09SSxACIi8KC0FkZHJlc3NCb29rEiAK" + 

-          "BnBlcnNvbhgBIAMoCzIQLnR1dG9yaWFsLlBlcnNvbkJFSAHCPkAKK0dvb2ds" + 

-          "ZS5Qcm90b2NvbEJ1ZmZlcnMuRXhhbXBsZXMuQWRkcmVzc0Jvb2sSEUFkZHJl" + 

-          "c3NCb29rUHJvdG9z");

+          "Chp0dXRvcmlhbC9hZGRyZXNzYm9vay5wcm90bxIIdHV0b3JpYWwi2gEKBlBl" + 

+          "cnNvbhIMCgRuYW1lGAEgAigJEgoKAmlkGAIgAigFEg0KBWVtYWlsGAMgASgJ" + 

+          "EisKBXBob25lGAQgAygLMhwudHV0b3JpYWwuUGVyc29uLlBob25lTnVtYmVy" + 

+          "Gk0KC1Bob25lTnVtYmVyEg4KBm51bWJlchgBIAIoCRIuCgR0eXBlGAIgASgO" + 

+          "MhoudHV0b3JpYWwuUGVyc29uLlBob25lVHlwZToESE9NRSIrCglQaG9uZVR5" + 

+          "cGUSCgoGTU9CSUxFEAASCAoESE9NRRABEggKBFdPUksQAiIvCgtBZGRyZXNz" + 

+          "Qm9vaxIgCgZwZXJzb24YASADKAsyEC50dXRvcmlhbC5QZXJzb25CAkgB");

       pbd::FileDescriptor.InternalDescriptorAssigner assigner = delegate(pbd::FileDescriptor root) {

         descriptor = root;

         internal__static_tutorial_Person__Descriptor = Descriptor.MessageTypes[0];

@@ -55,14 +52,10 @@
         internal__static_tutorial_AddressBook__FieldAccessorTable = 

             new pb::FieldAccess.FieldAccessorTable<global::Google.ProtocolBuffers.Examples.AddressBook.AddressBook, global::Google.ProtocolBuffers.Examples.AddressBook.AddressBook.Builder>(internal__static_tutorial_AddressBook__Descriptor,

                 new string[] { "Person", });

-        pb::ExtensionRegistry registry = pb::ExtensionRegistry.CreateInstance();

-        RegisterAllExtensions(registry);

-        global::Google.ProtocolBuffers.DescriptorProtos.CSharpOptions.RegisterAllExtensions(registry);

-        return registry;

+        return null;

       };

       pbd::FileDescriptor.InternalBuildGeneratedFileFrom(descriptorData,

           new pbd::FileDescriptor[] {

-          global::Google.ProtocolBuffers.DescriptorProtos.CSharpOptions.Descriptor, 

           }, assigner);

     }

     #endregion