it's really a bit weird that the windows configure has more options to
configure unix features than the unix one, even if some are just
workarounds for missing auto-detection.

unlike in configure.exe itself, -posix-iconv is now also understood for
symmetry with -gnu-iconv and -sun-iconv.

Change-Id: Ic15376e5822e43b998bd17f02c11e5dd0567dc2b
Reviewed-by: Lars Knoll <lars.knoll@qt.io>