[niepce] rust: fix build.rs if CARGO_TARGET_DIR is unknown

[Hubert Figuière <hub src gnome org>]

commit b7bea0214855c03950b411688c92747ab064dbcd
Author: Hubert Figuière <hub figuiere net>
Date:   Sat Feb 17 23:11:55 2018 -0500

    rust: fix build.rs if CARGO_TARGET_DIR is unknown

 build.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/build.rs b/build.rs
index b8a4887..cec2063 100644
--- a/build.rs
+++ b/build.rs
@@ -49,7 +49,7 @@ fn main() {
 
     // Use cbindgen to generate C bindings.
     let crate_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
-    let target_dir = env::var("CARGO_TARGET_DIR").unwrap();
+    let target_dir = env::var("CARGO_TARGET_DIR").unwrap_or(String::from("./target"));
     let mut target_file = PathBuf::from(target_dir);
     target_file.push("bindings.h");
     cbindgen::generate(&crate_dir)