plink: Fails with clang

sets.cpp:771:37: error: redefinition of 'i' with a different type:
'vector<Individual *>::iterator' (aka '__wrap_iter<pointer>') vs 'int'
      vector<Individual*>::iterator i = PP->sample.begin();
                                    ^
sets.cpp:703:12: note: previous definition is here
  for (int i=0; i<snpset.size(); i++)
           ^

Closes #492