cmd/gomobile: fix openal init test

Updates golang/go#19046

Change-Id: I0fc8fedc179d24ec9691528cab98857f3f9a4291
Reviewed-on: https://go-review.googlesource.com/37327
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
diff --git a/cmd/gomobile/init.go b/cmd/gomobile/init.go
index 4452c37..38faf94 100644
--- a/cmd/gomobile/init.go
+++ b/cmd/gomobile/init.go
@@ -195,14 +195,14 @@
 	if ndkRoot == "" || initOpenAL == "" {
 		return nil
 	}
-	sdkRoot := os.Getenv("ANDROID_HOME")
-	if sdkRoot == "" {
-		return nil
-	}
 	var cmake string
 	if buildN {
 		cmake = "cmake"
 	} else {
+		sdkRoot := os.Getenv("ANDROID_HOME")
+		if sdkRoot == "" {
+			return nil
+		}
 		var err error
 		cmake, err = exec.LookPath("cmake")
 		if err != nil {