Make virtualbox-guest-additions to compile

Currently official build for guest additions will fail due to some
unset/wrong set variables. This is an workaround for it - added KERN_VER
variable which is set to current kernel version, so this ebuild cannot
be run against new kernel version while executed on older/other kernel
version.

app-emulation/virtualbox-guest-additions/files/virtualbox-guest-additions-5-localconfig

@@ -0,0 +1,30 @@
+# -*- Makefile -*-
+#
+# Overwrite some default kBuild settings
+#
+
+#
+# Copyright (C) 2006-2008 Sun Microsystems, Inc.
+#
+# This file is part of VirtualBox Open Source Edition (OSE), as
+# available from http://www.virtualbox.org. This file is free software;
+# you can redistribute it and/or modify it under the terms of the GNU
+# General Public License as published by the Free Software Foundation,
+# in version 2 as it comes in the "COPYING" file of the VirtualBox OSE
+# distribution. VirtualBox OSE is distributed in the hope that it will
+# be useful, but WITHOUT ANY WARRANTY of any kind.
+#
+
+# don't build testcases to save time, they are not needed for the package
+VBOX_WITH_TESTCASES     :=
+VBOX_WITH_VALIDATIONKIT :=
+
+KBUILD_MSG_STYLE        := brief
+
+## paths, origin, hardening
+VBOX_WITH_HARDENING        := 2
+VBOX_WITH_ORIGIN           :=
+VBOX_ONLY_ADDITIONS        := 1
+
+## don't build with -Werror
+VBOX_WITH_WARNINGS_AS_ERRORS :=