The code and tests assume that the system supplied iconv() would
always use BOM in its output when asked to encode to UTF-16 (or
UTF-32), but apparently some implementations output big-endian
without BOM.  A compile-time knob has been added to help such
systems (e.g. NonStop) to add BOM to the output to increase
portability.

* bc/utf16-portability-fix:
  utf8: handle systems that don't write BOM for UTF-16