add DESTDIR support in make

Makefile

@@ -1,6 +1,7 @@
+PREFIX ?= /usr
+DESTDIR ?=
 BINARY=qulay
 SRC=main.go
-PREFIX=/usr
 
 .PHONY: all clean install uninstall deps
 
@@ -11,12 +12,12 @@ deps:
 	go mod tidy
 
 install: all
-	install -Dm755 $(BINARY) $(PREFIX)/bin/$(BINARY)
+	install -Dm755 $(BINARY) $(DESTDIR)$(PREFIX)/bin/$(BINARY)
-	@echo "qulay installed to $(PREFIX)/bin"
+	@echo "qulay installed to $(DESTDIR)$(PREFIX)/bin"
 
 uninstall:
-	rm -f $(PREFIX)/bin/$(BINARY)
+	rm -f $(DESTDIR)$(PREFIX)/bin/$(BINARY)
-	@echo "qulay removed from $(PREFIX)/bin"
+	@echo "qulay removed from $(DESTDIR)$(PREFIX)/bin"
 
 clean:
 	rm -f $(BINARY)