Update the build script

Cargo.toml

@@ -3,6 +3,7 @@ name = "google_speech"
 version = "0.1.0"
 authors = ["Uttarayan Mondal <uttarayan21@gmail.com>"]
 edition = "2018"
+build = "build.rs"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 

build.rs

@@ -0,0 +1,3 @@
+// OUT_DIR is only defined when there is a build script
+// https://github.com/rust-lang/cargo/issues/879#issuecomment-215491052
+pub fn main() {}

tests/tests.rs

@@ -8,7 +8,7 @@ fn test_hello_play() {
 #[test]
 fn test_hello_save() {
     let hello = Speech::new("Hello", Lang::en_us).unwrap();
-    unsafe { hello.save("target/hello.mp3").unwrap() };
+    unsafe { hello.save(concat!(env!("OUT_DIR"), "/hello.mp3")).unwrap() };
 }
 
 // #[test]