Fix build on windows-gnu

lzma-sys/build.rs

@@ -133,17 +133,29 @@ fn main() {
         }
 
         run(&mut cmd);
-        run(Command::new("make")
+        run(make()
                     .arg(&format!("-j{}", env::var("NUM_JOBS").unwrap()))
                     .current_dir(&dst.join("build")));
         // Unset DESTDIR or liblzma.a ends up in it and cargo can't find it
         env::remove_var("DESTDIR");
-        run(Command::new("make")
+        run(make()
                     .arg("install")
                     .current_dir(&dst.join("build/src/liblzma")));
     }
 }
 
+fn make() -> Command {
+    let mut cmd = Command::new("make");
+
+    // We're using the MSYS make which doesn't work with the mingw32-make-style
+    // MAKEFLAGS, so remove that from the env if present.
+    if cfg!(windows) {
+        cmd.env_remove("MAKEFLAGS").env_remove("MFLAGS");
+    }
+
+    return cmd
+}
+
 fn try_run(cmd: &mut Command) -> bool {
     println!("running: {:?}", cmd);
     t!(cmd.status()).success()