[Patch] Fix incorrect behavior of "[[=a=]]" in regex

[Jonathan Wakely]

On 2 October 2013 15:26, Tim Shen wrote:
> _BracketMatcher<>::_M_add_equivalence_class is misimplemented so I try
> `git blame regex_compiler.h`...that's me!
>
> Booted and tested under -m32, -m64.

This is OK to commit, thanks