Support custom cargo build dirs on MacOS.

[aldhsu]

Loving wezterm! I was attempting to add new lua hooks and ran into this problem on MacOs.

Expected

Clean build succeeds on MacOS

Actual

Fails to build.

 Caused by:
      No such file or directory (os error 2)
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[wez]

Thanks, but this shouldn't be needed if you follow the https://wezfurlong.org/wezterm/install/source.html instructions.
What's different about your environment/setup that results in you needing this?

[aldhsu]

Ah I see the problem. I just blindly made the folder without looking at what it's doing.
I should have read the error more closely.

I use a custom CARGO_TARGET_DIR in ~/.cargo/.config.toml:

[build]
target-dir = "/Users/ahsu/.cargo/builds/"

This is so deps are shared.

Might just be my configuration but I tried cargo build -vv --target-dir=target which forces the target dir over other config and printing out the "CARGO_TARGET_DIR" env var in the build script. "CARGO_TARGET_DIR" seems to always be empty? This comment rust-lang/cargo#9661 (comment) seems to suggest that not passing "CARGO_TARGET_DIR" from Cargo to build scripts is intentional currently.

A user can set it themselves though eg. CARGO_TARGET_DIR=target cargo build -vv but this will not come from cargo.toml by default.

The script currently always falls back to the default "target" on any systems building with a custom Cargo target directory.

A general solution would be to walk back from "OUT_DIR" which is passed into the build script.
I'm not sure I know enough to suggest a good solution but if the above will suffice I can amend the PR.

[aldhsu]

I've updated the PR to use OUT_DIR.