Look for the vendor backlight directory instead of hardcoding it

Hopefully this addresses the problem of the project not building on
systems with a different vendor backlight directory.

Makefile

@@ -2,10 +2,11 @@ CC = gcc
 CFLAGS += -Wall -Wextra -O3 -march=native
 
 prefix = /usr/local
-backlight_dir = /sys/class/backlight/intel_backlight/
-max_brightness = $(shell cat $(backlight_dir)max_brightness)
+backlight_dir = /sys/class/backlight
+vendor_backlight_dir = $(backlight_dir)/$(shell ls $(backlight_dir) | head -n1)
+max_brightness = $(shell cat $(vendor_backlight_dir)/max_brightness)
 
-DEFINES += -D BACKLIGHT_DIR=\"$(backlight_dir)\"
+DEFINES += -D BACKLIGHT_DIR=\"$(vendor_backlight_dir)/\"
 DEFINES += -D MAX_BRIGHTNESS=$(max_brightness)
 
 all: backlight_control