codecvt_error_category is not thread-safe

[Andrey Semashev]

The boost::filesystem3::codecvt_error_category uses unprotected function-local static variable of type codecvt_error_cat, which is not POD. One thread may half-initialize this value when the other thread will acquire and use a reference to the not yet constructed error category. The object must be protected with call_once or a similar technique.