Add ability to select libusb commit for built-in libusb

[Cu3PO42]

Hi,

first of all: thanks for your work on rusb! I'm quite excited to use it in a project. Unfortunately, I ran into these issues in libusb 1.0.24. As of current master, they appear to have been fixed, but no new release has been made yet. I understand that it's not desirable to ship a non-release libusb in a release libusb1-sys, but maybe this problem could be tackled by adding a generic way to select any commit of libusb that should be built.

I am currently not clear on how such a selection mechanism should work. A commit could be passed as an environment variable but that's not very ergonomic. If we can figure out a clean design, I'd be happy to send a pull request. One potential option would be to rely on rust-lang/cargo#9175, which has landed earlier this year, is now available as part of Rust stable, and allows a user to set environment variables in a .cargo/config.toml. This file can be configured per project, so it should provide a satisfactory solution.

EDIT: I have an experimental implementation of this at Cu3PO42/rusb. The primary issue remaining with this is the lack of a proper version string for the commit. I'm not sure what it's used for so I don't know if just using the commit would be fine.

[a1ien]

I think if you want use different version use with rusb it's build this version with /configure --prefix=<path_to_install>, run make install and use PKG_CONFIG_PATH=<path_to_install>/lib/pkgconfig for set place for search when run cargo build

[Cu3PO42]

I appreciate you getting back to me on this. Unfortunately I had to step away from that particular project for a while. I'm now back at it.

The way you describe should work, but would be mildly painful to execute, especially on Windows. libusb1-sys already provides such a great tool for building libusb cross-platform! I think it would be great if it would be possible to re-use that. With my commit, the following workflow is possible:

1. A project depends on libusb1-sys, either directly or indirectly through rusb.
2. The user sets LIBUSB_COMMIT=1a906274a66dd58bf81836db1306902d4a7dc185 in .cargo/config.toml.
3. Upon building, the specified commit of libusb will be fetched and built.

I think that is quite useful and if you're open to that functionality, I'm happy to send a PR. If you'd rather not include that, I will simply maintain this in a fork.

[a1ien]

This won't work because we upload only one version of libusb submodule to crates.io
And I against of downloading library while in build process. When build.rs was called all code mus be available and not download from internet. So as I say if you want different version build, build it you self.