Makefile: Add clean & install targets

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/Makefile b/Makefile
index 24a5cf4..ebf69f3 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,16 @@
 
 CC = gcc
 CFLAGS = -Wall -Wextra -Werror
+INSTALL = install
+prefix = /usr/local
+sbindir = ${prefix}/sbin
 
 all: inarp
+
+.PHONY: clean
+clean:
+	rm -f inarp
+
+.PHONY: install
+install: all
+	$(INSTALL) -m 0755 -D inarp $(DESTDIR)$(sbindir)/inarp