for x-builds, the host binaries and data may be in a different place
than the ones for the target.
we have fallback code for qt 4, so we can use the new variables
unconditionally.

this patch may be incomplete - there are still some uses which *may*
need changing.

Change-Id: Ia96c4ea99c5c7fc62bbe32d0283c82eef5c1eefd
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>