Add bindings for iconv calls

FreeBSD-likes all implement iconv:
- DragonflyBSD: https://github.com/DragonFlyBSD/DragonFlyBSD/blob/bbb35c81f71fe2a0880a1f8bb77876ee98b63338/include/iconv.h
- FreeBSD: https://github.com/freebsd/freebsd-src/blob/a6dc68c0e0f8a24ffaf0b4e78e58141ef7897047/include/iconv.h

NetBSD-likes:
- NetBSD: https://github.com/NetBSD/src/blob/81a39f60870b617d7fca299c126de6553d78cc5a/include/iconv.h
- OpenBSD doesn't implement it

macOS: apparently ships a conforming implementation as a separate
library:
https://stackoverflow.com/questions/57734434/libiconv-or-iconv-undefined-symbol-on-mac-osx/57734435#57734435

Linux:
- glibc: https://sourceware.org/git/?p=glibc.git;a=blob;f=iconv/iconv.h;h=fdddf53d99c3046ef9c280db01a425c2f499043e;hb=HEAD
- musl: https://git.musl-libc.org/cgit/musl/tree/include/iconv.h?id=455f96857f91d14e193219ca00969354a981c09c

1 files changed, 7 insertions, 0 deletions

diff --git a/build.rs b/build.rs
index 27cfb02401..ef43dfb788 100644
--- a/build.rs
+++ b/build.rs
@@ -10,6 +10,7 @@ fn main() {
     let const_extern_fn_cargo_feature =
         env::var("CARGO_FEATURE_CONST_EXTERN_FN").is_ok();
     let libc_ci = env::var("LIBC_CI").is_ok();
+    let target = env::var("TARGET").unwrap();
 
     if env::var("CARGO_FEATURE_USE_STD").is_ok() {
         println!(
@@ -82,6 +83,12 @@ fn main() {
         }
         println!("cargo:rustc-cfg=libc_const_extern_fn");
     }
+
+    // For unknown reason, libiconv can't be linked by adding #[link(name = iconv)] to
+    // a macOS-specific struct, so we do the linking here.
+    if target.contains("-apple-") {
+        println!("cargo:rustc-link-lib=iconv");
+    }
 }
 
 fn rustc_minor_nightly() -> Option<(u32, bool)> {