https://bugs.gentoo.org/298988

--- a/vboxguest/Makefile
+++ b/vboxguest/Makefile
@@ -167,6 +167,7 @@ VBOXMOD_DEFS = \
 	IN_GUEST_R0 \
 	IN_MODULE \
 	RT_WITH_VBOX \
+	LOG_USE_C99 \
 	VBGL_VBOXGUEST \
 	VBOX_WITH_HGCM
 ifeq ($(KERN_VERSION),24)