MacOS iconv linkage misdetection (also deprecation warnings)

Forwarding https://github.com/nabijaczleweli/cargo-update/issues/176 and https://github.com/nabijaczleweli/cargo-update/issues/186 – MacOS/M1 and previous ports:

Cargo-update has

git2 = "0.11"

set, and a user got

  = note: Undefined symbols for architecture arm64:
            "_iconv", referenced from:
                _git_path_iconv in liblibgit2_sys-c72467cf4143557b.rlib(path.o)
               (maybe you meant: _git_path_iconv_init_precompose, _git_path_iconv , _git_path_iconv_clear )
            "_iconv_open", referenced from:
                _git_path_direach in liblibgit2_sys-c72467cf4143557b.rlib(path.o)
                _git_path_iconv_init_precompose in liblibgit2_sys-c72467cf4143557b.rlib(path.o)
                _git_path_diriter_init in liblibgit2_sys-c72467cf4143557b.rlib(path.o)
            "_iconv_close", referenced from:
                _git_path_direach in liblibgit2_sys-c72467cf4143557b.rlib(path.o)
                _git_path_iconv_clear in liblibgit2_sys-c72467cf4143557b.rlib(path.o)
                _git_path_diriter_free in liblibgit2_sys-c72467cf4143557b.rlib(path.o)
                _git_path_dirload in liblibgit2_sys-c72467cf4143557b.rlib(path.o)
          ld: symbol(s) not found for architecture arm64

Start by reviewing the git2 = "0.11" dependency and reproducing the reported macOS/M1 linker failure involving iconv symbols. Compare the forwarded cargo-update reports and determine how iconv linkage is detected; done means the affected macOS build links successfully without the reported deprecation warnings.