relax requirements for vector==

[Benjamin Kosnik]

Instead of relaxing requirements, perhaps a more sane thing to do is
to make the concept checking code a flag you have to turn on (at
configure time?), and not something that is forced on everybody's code.

There's been some talk about updating the concept checking code to
something recent, and enabling it at configure time. 

Anybody who has strong feelings, please voice them...

-benjamin