using the Makefile target separator to determine the host system is of
course not a terribly good idea. the correct variable to query would be
DIR_SEPARATOR, which always reflects the host. however, a direct os
query is less obscure in this case.

Task-number: QTBUG-54346
Change-Id: I6bc1372a7c9a84a241a97f9f767f046b026411c3
Reviewed-by: Kai Koehne <kai.koehne@qt.io>