configure: add "we mean it" blurb

like all qt_*.prf files (exceptions prove the rule), the configure
system counts as private api.

Change-Id: Iea3057445e430029ecd8449e604e2d5c499ae10a
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

mkspecs/features/qt_configure.prf

@@ -1,3 +1,13 @@
+#
+#  W A R N I N G
+#  -------------
+#
+# This file is not part of the Qt API.  It exists purely as an
+# implementation detail.  It may change from version to version
+# without notice, or even be removed.
+#
+# We mean it.
+#
 
 QT_CONFIGURE_REPORT =
 QT_CONFIGURE_NOTES =