Building on FreeBSD: icu-config vs. pkg-config

I get the following error when compiling `rust_icu_sys` on FreeBSD:

```
error: failed to run custom build command for `rust_icu_sys v2.0.0`

Caused by:
process didn't exit successfully: `/usr/home/kaj/proj/r4s/target/debug/build/rust_icu_sys-8f83e049d2164566/build-script-build` (exit status: 1)
--- stdout
cargo:rustc-cfg=feature="renaming"
cargo:rustc-cfg=feature="use-bindgen"
cargo:rustc-cfg=feature="icu_config"

--- stderr
Error: while getting ICU version; is icu-config in $PATH?

Caused by:
0: could not execute command: pkg-config
1: No such file or directory (os error 2)
```

The error message mentions `icu-config` (which I do have on my `$PATH`), but the command it tried to execute is `pkg-config`, which is not available on FreeBSD. Should the be a different implementation of `struct ICUConfig` for FreeBSD (and possibly other systems that use `icu-config` instead of `pkg-config`) ? Is there such an implemention that just failed to be selected in my compilation (but if so, I didn't find it in the source) ?