prefer /get variants of qmake properties

we care for the current value, not the installed one. this only matters
for developers of qt, but that's sufficiently many, i'd think. :)
it wouldn't be nice to "litter" all variable references with the "/get"
suffix, so do it centrally (even if that's a tad slower).

Change-Id: I620f3da9185e7d2dd7bd4744f28d59d561b79132
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>