Makefile: use PREFIX for install

Author: nicoulaj

makefile

@@ -1,10 +1,12 @@
+PREFIX ?= /usr/local
+
 cshatag:
 	gcc cshatag.c -l crypto -o cshatag
 
 install:
-	cp -v cshatag /usr/local/bin
-	mkdir -v /usr/local/share/man/man1 2> /dev/null || true
-	cp -v cshatag.1 /usr/local/share/man/man1
+	cp -v cshatag ${PREFIX}/bin
+	mkdir -v ${PREFIX}/share/man/man1 2> /dev/null || true
+	cp -v cshatag.1 ${PREFIX}/share/man/man1
 
 clean:
-	rm -f cshatag
+	rm -f cshatag