configure: Respect -continue in qtConfParseCommandLine

If configure is called with -continue, it should not stop processing
command line arguments after encountering an invalid one.

Example:
  configure ... -continue -quack -no-feature-gui
would ignore everything after -quack.

Change-Id: Ia5f0cb13414c9c0c7246ff0c72f8e935fe6dca3c
Fixes: QTBUG-72912
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

mkspecs/features/qt_configure.prf

@@ -344,7 +344,10 @@ defineTest(qtConfParseCommandLine) {
 
         isEmpty(type) {
             qtConfAddError("Unknown command line option '$$c'.")
-            return()
+            equals(config.input.continue, yes): \
+                next()
+            else: \
+                return()
         }
 
         call = "qtConfCommandline_$${type}"