Change some QNX defaults

Make REDUCE_EXPORTS the default for QNX.  This is what the Linux builds
use.  The Windows builds should too.

Turn on ICU detection for QNX.  QNX has ICU.

Task-number: QTBUG-52578
Change-Id: Ie65c6ff03c4eecf361727b3b6026338f686d9749
Reviewed-by: Dan Cape <dcape@qnx.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>