cmState: Make it possible to order cmState::Snapshot.

Source/cmLinkedTree.h

@@ -128,6 +128,13 @@ public:
         }
       return this->Position <= this->Tree->Data.size();
     }
+
+    bool StrictWeakOrdered(iterator other) const
+    {
+      assert(this->Tree);
+      assert(this->Tree == other.Tree);
+      return this->Position < other.Position;
+    }
   };
 
   iterator Root() const

Source/cmState.cxx

@@ -1411,3 +1411,9 @@ void cmState::Directory::ClearCompileOptions()
                this->DirectoryState->CompileOptionsBacktraces,
                this->Snapshot_.Position->CompileOptionsPosition);
 }
+
+bool cmState::Snapshot::StrictWeakOrder::operator()(
+    const cmState::Snapshot& lhs, const cmState::Snapshot& rhs) const
+{
+  return lhs.Position.StrictWeakOrdered(rhs.Position);
+}

Source/cmState.h

@@ -74,9 +74,16 @@ public:
 
     Directory GetDirectory() const;
 
+    struct StrictWeakOrder
+    {
+      bool operator()(const cmState::Snapshot& lhs,
+                      const cmState::Snapshot& rhs) const;
+    };
+
   private:
     friend class cmState;
     friend class Directory;
+    friend struct StrictWeakOrder;
     cmState* State;
     cmState::PositionType Position;
   };