feat: added completely classical Makefile

Makefile

@@ -0,0 +1,13 @@
+
+PREFIX=/usr/local
+
+all: filter-archive
+
+clean:
+	if [ -e ./filter-archive ] ; then rm filter-archive ; fi
+
+filter-archive: clean
+	go build
+
+install: filter-archive
+	cp -av ./filter-archive $(PREFIX)/libexec/opensmtpd