Allow txt files as ExtraSources in object library targets

Necessary for the Xcode generator, in which the CMakeLists.txt
files are added as sources merely for convenient access, without
any associated custom command.

Source/cmGeneratorTarget.cxx

@@ -74,7 +74,10 @@ void cmGeneratorTarget::ClassifySources()
     else
       {
       this->ExtraSources.push_back(sf);
-      if(isObjLib) { badObjLib.push_back(sf); }
+      if(isObjLib && cmSystemTools::LowerCase(sf->GetExtension()) != "txt")
+        {
+        badObjLib.push_back(sf);
+        }
       }
     }