diff -Nurp VirtualBox-5.1.0.p17/src/VBox/Additions/common/VBoxGuestLib/Makefile.kmk VirtualBox-5.1.0.p20/src/VBox/Additions/common/VBoxGuestLib/Makefile.kmk
--- VirtualBox-5.1.0.p17/src/VBox/Additions/common/VBoxGuestLib/Makefile.kmk	2016-07-12 17:47:38.000000000 +0300
+++ VirtualBox-5.1.0.p20/src/VBox/Additions/common/VBoxGuestLib/Makefile.kmk	2016-07-13 19:34:18.617337885 +0300
@@ -39,10 +39,12 @@ LIBRARIES += \
 	VBoxGuestR3Lib \
 	VBoxGuestR3LibShared
 ifndef VBOX_ONLY_VALIDATIONKIT
+ ifndef VBOX_USE_SYSTEM_XORG_HEADERS
  if1of ($(KBUILD_TARGET), freebsd linux netbsd openbsd)
   LIBRARIES += \
   	VBoxGuestR3LibXFree86
  endif
+ endif
  if1of ($(KBUILD_TARGET), freebsd linux netbsd openbsd solaris)
   LIBRARIES += \
   	VBoxGuestR3LibXOrg