Don't unconditionally link libiconv on macOS.

[goffrie]

This adds #[link(name = "iconv")] to just the iconv symbols so that the
library is only linked if the symbols are used.

#2037 added a build.rs directive to always link libiconv on Apple platforms,
but that shouldn't be necessary. With this change, we can see using otool -L
that a binary that uses an iconv symbol links libiconv, but other binaries
don't.

Note that this can only be seen with a rustc prior to nightly-2021-03-09, as
nightly-2021-03-10 includes rust-lang/rust#82731 which
includes #2037, therefore unconditionally linking all Rust binaries to
libiconv.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[JohnTitor]

r? @Amanieu

[Amanieu]

If CI is happy then it's fine.

@bors r+

[bors]

📌 Commit 28f07a4 has been approved by Amanieu

[bors]

⌛ Testing commit 28f07a4 with merge e584862...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: Amanieu
Pushing e584862 to master...

[winterqt]

@goffrie It seems that libiconv is still unconditionally linked to all Rust binaries on macOS:

$ rustc --version
rustc 1.56.0-nightly (1f0a591b3 2021-07-30)
$ cat main.rs
fn main() {}
$ rustc main.rs
$ otool -L main
main:
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1292.100.5)
	/usr/lib/libresolv.9.dylib (compatibility version 1.0.0, current version 1.0.0)
	/usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)

The libc version in this compilers std is 0.2.98, which includes this patch. Do you have any idea why this is still happening?