--- a/Makefile	2019-08-20 14:22:17.158034121 -0500
+++ b/Makefile	2019-08-20 14:26:50.731987080 -0500
@@ -26,8 +26,8 @@ ifdef HAS_VBE
 TARGETS += monitor-get-edid-using-vbe
 endif
 
-CPPFLAGS = -I.
-CFLAGS = -O -Wall -g
+CPPFLAGS := -I. $(CPPFLAGS)
+CFLAGS := -O -Wall -g $(CFLAGS)
 OBJS = monitor-get-edid-using-vbe.c vbe.o
 
 monitor-get-edid-using-vbe: LDFLAGS += -lx86