Really fix handling of generator.

prepare.py

@@ -164,7 +164,7 @@ def main(argv=None):
     argparser.add_argument(
         '-f', '--force', help="Force preparation, even if working directory already exist.", action='store_true')
     argparser.add_argument(
-        '-G', '--generator', help="CMake build system generator (default: Unix Makefiles, use cmake -h to get the complete list).", default='Unix Makefiles', dest='generator')
+        '-G', '--generator', help="CMake build system generator (default: let CMake choose, use cmake -h to get the complete list).", default=None, dest='generator')
     argparser.add_argument(
         '-L', '--list-cmake-variables', help="List non-advanced CMake cache variables.", action='store_true', dest='list_cmake_variables')
     argparser.add_argument(
@@ -226,8 +226,9 @@ def main(argv=None):
         target = PythonRaspberryTarget()
     else:
         target = DesktopTarget()
-    if target.generator is None:
-        additional_args += ["-G", args.generator]
+    if args.generator is not None:
+        target.generator = args.generator
+
     if args.clean or args.veryclean:
         if args.veryclean:
             target.veryclean()