libgit2 0.28.x is shipped by latest Yocto Dunfell. According to https://github.com/rust-lang/git2-rs/issues/458#issuecomment-522567539, there are no compatibility guarantees between libgit2-sys and arbitrary system libgit2 versions, so better keep this turned off. Fixes "invalid version 3 on git_proxy_options" during build. Further references: * https://github.com/rust-lang/git2-rs/issues/458 * https://bugs.gentoo.org/707746#c1