cmGlobalGenerator: Fix value type pushed into autogens vector

The parent commit changed the AutogensType::value_type to be
"std::pair<cmQtAutoGenerators,cmTarget const*>" but our std::make_pair
call returns "std::pair<cmQtAutoGenerators,cmTarget*>".  Construct the
value_type directly instead of using make_pair.  Otherwise the Sun 5.9
compiler complains

 ".../Source/cmGlobalGenerator.cxx", line 1281: Error:
 Formal argument x of type "const std::pair<cmQtAutoGenerators, const cmTarget*>&"
 in call to "std::vector<std::pair<cmQtAutoGenerators, const cmTarget*> >
 ::push_back(const std::pair<cmQtAutoGenerators, const cmTarget*>&)" is being
 passed "std::pair<cmQtAutoGenerators, cmTarget*>".
Merge topic 'constify' into next

ae6fc55 cmGlobalGenerator: Fix value type pushed into autogens vector