$NetBSD$

Make the Makefile honor CC/CFLAGS/LDFLAGS.

--- Makefile.orig	2018-03-02 01:52:07.000000000 +0000
+++ Makefile
@@ -1,6 +1,6 @@
-CC	:= gcc
-CFLAGS	:= -O2 -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-parameter -g2
-LDFLAGS := -luuid
+CC	?= gcc
+CFLAGS	?= -O2 -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-parameter -g2
+LDFLAGS ?= -luuid
 
 all: vhdtool
 
