FYI: 3_4-branch mt_allocator synched with mainline

[Benjamin Kosnik]

>the below is the full ChangeLog. To my best knowledge, there are now
>only two pending issues in this area
>
> 1- Further improved alignment (from 4 -> 8 on 32-bit machines and from
>    8 -> 16 on 64-bit machines)
> 2- Static inizialization troubles mentioned by Brad Spencer.

Thanks very much for this Paolo! This is great: we've been needing to
sync this up.

I'd planned on looking at item 2 in the next couple of days. Isn't #1
pretty simple? If so, it should go in.

>Otherwise, the code seems in pretty good shape, a serious option for
>mainline and 3_4 too.

Yep. I'd like to look at trying to switch over to this, at least on some
platforms, in the next week.

-benjamin