phobos unittests not passing with dmd built by clang

[Trass3r]

>> The autotester hasn't revealed this cause it still uses the old g++4.2  
>> on OSX.
>> Apple switched to Clang just recently.
>
> "gcc" on OS X 10.7, which I am using, defaults to using clang. So I am  
> using clang on a regular basis, and have not seen issues.

It's a really small corner case.
Here's a fix: https://github.com/D-Programming-Language/dmd/pull/668