Unvendor system dependencies

[xhochy]

Comment:

• zlib: This depends on an implementation of Add ZLIB_DIR config option, and don't vendor zlib or zlib-ng automatically rust-lang/libz-sys#147 as libz-sys automatically vendors for most systems.
• mimalloc: This only supports vendoring via libmimalloc-sys: https://github.com/purpleprotocol/mimalloc_rust/blob/934a2fb453705a7c7b42890ba36f665bed5e53d6/libmimalloc-sys/build.rs#L3
• libgit2: We can force devendoring by setting LIBGIT2_NO_VENDOR: https://github.com/rust-lang/git2-rs/blob/19b6d056bfbfb86c21b494a9974e02e7c329d760/libgit2-sys/build.rs#L35C41-L35C58
• jemalloc can be unverdored by setting JEMALLOC_OVERRIDE: https://github.com/gnzlbg/jemallocator/blob/c27a859e98e3cb790dc269773d9da71a1e918458/jemalloc-sys/build.rs#L96
• zstd: We can force devendoring by setting ZSTD_SYS_USE_PKG_CONFIG: https://github.com/gyscos/zstd-rs/blob/90bc5928b006cd1a40b9bf00ca244085f1c3c4dc/zstd-safe/zstd-sys/build.rs#L261
• lz4 lz4-sys doesn't support devendoring at all