--- a/Makefile	2019-05-22 19:54:23.980451289 +0800
+++ b/Makefile	2019-06-10 10:32:58.054675929 +0800
@@ -1,7 +1,7 @@
-CC=gcc
-CFLAGS=-O3 -Wall -Wextra -Wno-unknown-pragmas
-LDFLAGS=
-LIBS=-lm -lpthread -lz
+CC?=gcc
+CFLAGS?=-O3 -Wall -Wextra -Wno-unknown-pragmas
+LDFLAGS?=
+LIBS?=-lm -lpthread -lz
 ZOPFLI=zopfli/src/zopfli/
 ZOP=deflate.o blocksplitter.o tree.o lz77.o cache.o hash.o util.o squeeze.o katajainen.o