doc/progs: don't try building cgo tests if CGO_ENABLED=0

R=golang-dev, jsing, r
CC=golang-dev
https://golang.org/cl/14523056

doc/progs/run

@@ -50,6 +50,9 @@ fi
 if [ "$goos" == "openbsd" ]; then
 	c_go_cgo="cgo1 cgo2"
 fi
+if [ "$CGO_ENABLED" != 1 ]; then
+	c_go_cgo=""
+fi
 
 timeout="
 	timeout1