[Bug libstdc++/55728] std::bad_function_call has misleading what() result

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55728

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-01-03 16:35:46 UTC ---
Yes, I would have done it already but was taking a break from writing any code
over xmas!

This one needs a new export from the lib, but that's OK, I bumped the version
to 6.0.18 recently anyway.