the code got factored out to an own toolchain.prf file, which is
load()ed from default_pre.prf, so no change at first.
however, on mac, we shadow toolchain.prf, and make it load() sdk.prf
first.
a side effect, it has become harder to disable the use of an sdk
altogether: putting CONFIG-=sdk into a project file or the qmake
command line has no effect now. instead, it's possible to put it into
.qmake.{conf,cache}.
to make it simpler again, it's conceivable to finally add qmake -pre,
which would allow setting variables before default_pre.prf is executed.
take 2: there was nothing wrong with the original patch, but in 5.8,
CONFIG+=simulator_and_device moved from qconfig.pri to various prf files
that would do it according to the simulator_and_device configure
feature, which would be way too late for the "pulled ahead" sdk.prf
loading. as simulator_and_device is now gone entirely, it is safe to
re-apply this patch (mostly) as-is.
Task-number: QTBUG-56144
Change-Id: I6cf484982eaed8af39f7a539c60f5a087a299914
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
