patch: mt_allocator.h

[Paolo Carlini]

Paolo Carlini wrote:

> I will play a bit myself with the current one (looks ok)

... however, as I feared, the patch, as-is, breaks the build: the 
library doesn't link anymore, many multiply defined symbols.

When working on the allocators, "check & check-performance" means, 
*always*, on a library built passing the meaningful 
--enable-libstdcxx-allocator= option.

Paolo.