use pkg-config instead of the removed imlib2-config

Index: makefile
--- makefile.orig
+++ makefile
@@ -2,7 +2,7 @@ include commands.mk
 
 OPTS    := -O3
 CFLAGS  := -std=c99 $(OPTS) $(shell pkg-config libheif --cflags) -fPIC -Wall
-LDFLAGS := $(shell imlib2-config --libs) $(shell pkg-config libheif --libs)
+LDFLAGS := $(shell pkg-config libheif imlib2 --libs)
 
 
 SRC = $(wildcard *.c)
