Don't terminate if GOPATH env var is unset

From version 1.8 onwards a default GOPATH is assumed, so use that if one
isn't explicitly set.

Author: Jeff Peeler

cmd/kubebuilder/main.go

@@ -17,6 +17,7 @@ limitations under the License.
 package main
 
 import (
+	gobuild "go/build"
 	"log"
 	"os"
 	"path/filepath"
@@ -38,7 +39,7 @@ func main() {
 	util.CheckInstall()
 	gopath := os.Getenv("GOPATH")
 	if len(gopath) == 0 {
-		log.Fatal("GOPATH not defined")
+		gopath = gobuild.Default.GOPATH
 	}
 	util.GoSrc = filepath.Join(gopath, "src")