Actively discard return value of qtConfGetNextCommandlineArg

Calling a qmake replace function without assignment is undefined
behavior.

This amends 11ae0e77.

Change-Id: Ie716f295275d1ba79a217745b332a8eca04b355d
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

mkspecs/features/qt_configure.prf

@@ -338,7 +338,7 @@ defineTest(qtConfParseCommandLine) {
                 qtConfAddWarning("Command line option -skip is only effective in top-level builds.")
                 skipOptionWarningAdded = 1
             }
-            $$qtConfGetNextCommandlineArg()
+            val = $$qtConfGetNextCommandlineArg()
             next()
         }