cmDefinitions: Avoid a find-then-insert when setting variables

Searching the map is not necessary.

Source/cmDefinitions.cxx

@@ -50,16 +50,7 @@ cmDefinitions::SetInternal(const std::string& key, Def const& def)
   if(this->Up || def.Exists)
     {
     // In lower scopes we store keys, defined or not.
-    MapType::iterator i = this->Map.find(key);
-    if(i == this->Map.end())
-      {
-      i = this->Map.insert(MapType::value_type(key, def)).first;
-      }
-    else
-      {
-      i->second = def;
-      }
-    return i->second;
+    return (this->Map[key] = def);
     }
   else
     {