Patch for LWG issue #2145

[David Blaikie]

On Tue, Aug 20, 2013 at 11:11 AM, 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)

Curious - I haven't dealt with constexpr much, but could you explain
further why this is untestable yet is a useful/meaningful change to
make?

- David