If $GOROOT is defined, Makefile should respect the value, fix #8

Makefile

@@ -1,9 +1,17 @@
 .PHONY: all clean install uninstall deps
 
+PREFIX = /usr/local
+
+ifeq ($(GOROOT),)
 GOBUILD = go build
 GOGET = go get -d -v
 GOGET_UPDATE = go get -d -u -v
-PREFIX=/usr/local
+else
+GOBUILD = $(GOROOT)/bin/go build
+GOGET = $(GOROOT)/bin/go get -d -v
+GOGET_UPDATE = $(GOROOT)/bin/go get -d -u -v
+endif
+
 ifeq ($(shell uname),Darwin)
 CONFDIR = /usr/local/etc/dns-over-https
 else