--- VirtualBox-5.2.2/src/VBox/Installer/linux/Makefile.include.header.orig	2017-11-25 14:42:43.336204931 +0100
+++ VirtualBox-5.2.2/src/VBox/Installer/linux/Makefile.include.header	2017-11-25 14:43:00.666146939 +0100
@@ -136,7 +136,7 @@
 endif # neq($(KERNELRELEASE),)
 
 # Kernel build folder
-KERN_DIR := /lib/modules/$(KERN_VER)/build
+KERN_DIR ?= /lib/modules/$(KERN_VER)/build
 ifneq ($(shell if test -d $(KERN_DIR); then echo yes; fi),yes)
  $(error Error: unable to find the headers of the Linux kernel to build against. \
           Specify KERN_VER=<version> (currently $(KERN_VER)) and run Make again)