--- Makefile.orig	2018-06-18 00:15:09 UTC
+++ Makefile
@@ -1,14 +1,23 @@
-CC=gcc
-CFLAGS=-g -Wall -O2 -Wno-unused-function
-BINDIR=/usr/local/bin
 
-all:seqtk
+# Respect user's build environment by using ?= to set defaults for variables
+# not set in the env or make arguments.  Use += to add essential args.
 
-seqtk:seqtk.c khash.h kseq.h
-		$(CC) $(CFLAGS) seqtk.c -o $@ -lz -lm
+CC	?= gcc
+CFLAGS	?= -g -Wall -O2 -Wno-unused-function
+LDFLAGS	+= -lz -lm
+PREFIX	?= /usr/local
+MKDIR	?= mkdir
+INSTALL	?= install
+# DESTDIR is empty by default to install straight to PREFIX
 
-install:all
-		install seqtk $(BINDIR)
+all: seqtk
 
+seqtk: seqtk.c khash.h kseq.h
+	$(CC) $(CFLAGS) seqtk.c -o $@ $(LDFLAGS)
+
+install: all
+	$(MKDIR) -p $(DESTDIR)$(PREFIX)/bin
+	$(INSTALL) -c -m 0755 seqtk $(DESTDIR)$(PREFIX)/bin
+
 clean:
-		rm -fr gmon.out *.o ext/*.o a.out seqtk trimadap *~ *.a *.dSYM session*
+	rm -fr gmon.out *.o ext/*.o a.out seqtk trimadap *~ *.a *.dSYM session*