Crates dependent on `libgit2-sys` fail to link when `libiconv` is installed via MacPorts

How to reproduce:

1. Use a machine running macOS 11 with MacPorts installed
2. Install libiconv using MacPorts: sudo port install libiconv
   Note: many common ports depend on libiconv so it is usually installed indirectly
3. Build a crate which depends (directly or indirectly) on libgit2-sys e.g. cargo:

   git clone https://github.com/rust-lang/cargo.git
   cd cargo
   cargo build
   

Expected result:

The crate builds successfully

Actual result:

• In my case cargo is cloned to /Users/david/Code/cargo
• I believe this issue is not specific to the arm64 architecture

error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-arch" "arm64" "-L" "/Users/david/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib" "/Users/david/Code/cargo/target/debug/deps/cargo.13dqm6cx88ih8d7v.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.13fag48fin0k9e60.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.13k5sy4x5ct7uc76.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.13mvacfztkpqoulz.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.17o1oyr8zzw4fskw.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.17velqna0uus6jym.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.1bqtvubbafsow4tm.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.1bzcfj2o6vziubl8.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.1cdjzrhuo6413him.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.1d13nnm6m5msm11z.rcgu.o" "/Users/david/Code/cargo/target/debug/deps/cargo.1di4cbgimehpmvur.rcgu.o" 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"/Users/david/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libcfg_if-8895b042fd9d6a9d.rlib" "/Users/david/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/liblibc-cacf286c763defc4.rlib" "/Users/david/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/liballoc-348755f31f5f7b8f.rlib" "/Users/david/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/librustc_std_workspace_core-00b1c5d56c8c46f3.rlib" "/Users/david/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libcore-59ed52fd3946b1c5.rlib" "/Users/david/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libcompiler_builtins-d11cefe128d6b458.rlib" "-framework" "CoreFoundation" "-liconv" "-framework" "Security" "-framework" "CoreFoundation" "-lssl" "-lcrypto" "-lclang_rt.osx" "-framework" "Security" "-framework" "CoreFoundation" "-lz" "-lSystem" "-lresolv" "-lc" "-lm"
  = note: Undefined symbols for architecture arm64:
            "_iconv", referenced from:
                _git_path_iconv in liblibgit2_sys-2871391079a10068.rlib(path.o)
               (maybe you meant: _git_path_iconv_init_precompose, _git_path_iconv_clear , _git_path_iconv )
            "_iconv_close", referenced from:
                _git_path_iconv_clear in liblibgit2_sys-2871391079a10068.rlib(path.o)
            "_iconv_open", referenced from:
                _git_path_iconv_init_precompose in liblibgit2_sys-2871391079a10068.rlib(path.o)
          ld: symbol(s) not found for architecture arm64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

Additional notes:

This is a reopening of #180 which has started occurring again on macOS 11, due to the following change:

According to the macOS Big Sur 11 Beta Release Notes, "the system ships with a built-in dynamic linker cache of all system-provided libraries. As part of this change, copies of dynamic libraries are no longer present on the filesystem."

This presumably causes the libiconv from MacPorts to take priority over the built-in one, as the built-in one is no longer provided in /usr/lib (which is prioritised over /opt/local/lib).

Start by reproducing the issue on macOS 11 with MacPorts libiconv installed, using the cargo clone and cargo build command shown. Inspect libgit2-sys build and linking configuration; done means crates depending on it, including cargo, link successfully in this environment without the reported error.