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

Makefile

@@ -1,13 +1,21 @@
 .PHONY: all clean install uninstall deps
 
-GOBUILD=go build
+PREFIX = /usr/local
-GOGET=go get -d -v
+
-GOGET_UPDATE=go get -d -u -v
+ifeq ($(GOROOT),)
-PREFIX=/usr/local
+GOBUILD = go build
-ifeq ($(shell uname),Darwin)
+GOGET = go get -d -v
-	CONFDIR=/usr/local/etc/dns-over-https
+GOGET_UPDATE = go get -d -u -v
 else
-	CONFDIR=/etc/dns-over-https
+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
+CONFDIR = /etc/dns-over-https
 endif
 
 all: doh-client/doh-client doh-server/doh-server