Consume the -skip option in qtbase/configure

...and yield a warning that -skip has no effect in a qtbase build.

This is consistent with configure's help output and enables us to
always pass "-skip qtwhatnot", whether we're calling top-level or
qtbase configure.

Change-Id: Ie5b0791a6000d1d78b1367658ad86a92b2ec6a6a
Fixes: QTBUG-71253
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

mkspecs/features/qt_configure.prf

@@ -333,6 +333,15 @@ defineTest(qtConfParseCommandLine) {
             type = boolean
         }
 
+        isEmpty(type):contains(opt, "skip") {
+            isEmpty(skipOptionWarningAdded) {
+                qtConfAddWarning("Command line option -skip is only effective in top-level builds.")
+                skipOptionWarningAdded = 1
+            }
+            $$qtConfGetNextCommandlineArg()
+            next()
+        }
+
         isEmpty(type) {
             qtConfAddError("Unknown command line option '$$c'.")
             return()