libstdc++: Fix versioned namespace build

     libstdc++-v3/ChangeLog:

             * libsupc++/eh_ptr.cc (exception_ptr(__safe_bool)): Define if
             _GLIBCXX_EH_PTR_COMPAT is defined.
             (exception_ptr::_M_safe_bool_dummy()): Likewise.
             (exception_ptr::operator!()): Likewise.
             (exception_ptr::opeerator __safe_bool()): Likewise.

Ok to commit ?

François