Patch for LWG issue #2145

[Howard Hinnant]

On Aug 20, 2013, at 2:11 PM, Marshall Clow <mclow.lists at gmail.com> wrote:

> http://cplusplus.github.io/LWG/lwg-defects.html#2145
> 
> Mark the constructor for std::error_category as inline and constexpr.
> Leave the (existing, out-of-line, non-constexpr) in the dylib for compatibility with existing programs)
> No tests, because I don't know how to test it (given that error_category is both an abstract class and has a user-defined destructor)

Awesome job Marshall.  I can find nothing to complain about.  Please commit.

Thanks,
Howard