Some unicode bson test are failing due to libstdc++'s iconv codecvt

[chrismanning]

To workaround libstdc++'s lack of header <codecvt> I've used the codecvt specialisation in <ext/codecvt_specializations.h> which wraps iconv.

BsonTest.FileTest17 and BsonTest.FileTest56 are failing when using this.

FileTest17 contains valid unicode, codecvt reports error. FileTest56 contains invalid unicode, codecvt reports success.

Possible issue with iconv itself. More investigation needed.