[v3] default linux allocator

[Benjamin Kosnik]

>You need to justify why you want to move this to the 3.4 branch, unless 
>it is fixing a regression.

Of course. 

Do you think this is something that the linux vendors should just agree
on, to include a --enable-libstdcxx-allocator=mt along with
--enable-__cxa_atexit for toolchains?

Personally, I would prefer to set the defaults in the configury.

-benjamin