Add support for QNX iconv implementation

Add support for QNX iconv implementation

QNX implements the POSIX version of iconv (with non-const function
signatures). However, it is still necessary to link with libiconv, unlike most
cases. Also, its iconv_open does not know how to handle an empty string.

Change-Id: I8654703e46b9c64503aca5521ce7fae1c97d7968
Reviewed-by: Mark Brand <mabrand@mabrand.nl>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>