in its current form, it was introduced only in 5.7, mostly as a side
effect of -external-hostbindir (which is now handled differently).
it only ever worked for the macOS and MinGW specs, as a side effect of
them supporting -sdk and -device-option (for good reasons), and was
supported only by the unix configure. it's not believed to be really
useful and complicates matters somewhat, so get rid of it again.

should it ever become actually relevant, it can be re-introduced
properly, probably along with a -host-sdk option for macOS.

Change-Id: Ib078469ea39deb821c7b6a8c67fda9e1a95fedf5
Reviewed-by: Lars Knoll <lars.knoll@qt.io>