Clean "system" calls to build&run gn and ninja

There was a time when gn build was called during make step,
however this was not working with recursive qmake calls.
Clean up leftovers and fix path and warnings like
"Conditional must expand to exactly one word." during builds.

Change-Id: I8546520345a5f89ee829558fa0fd9183587848b3
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>