eventually it turns out that our current
EIGEN_WORK_AROUND_QT_BUG_CALLING_WRONG_OPERATOR_NEW_FIXED_IN_QT_4_5
is the right way to go for allowing placement new on a class having
overloaded operator new. Qt 4.5 won't add the :: prefix. (I still do not
understand how you can distinghish a placement new from an overloaded
operator new taking an allocator as argument...)