Use system default location for libzfs on illumos

Makes compilation successful and running `cargo run --example sync-hang`
in zfs-core/ works. No further testing done.

zfs-core-sys/build.rs

@@ -60,8 +60,9 @@ fn main() {
  let lookup_with: Option<Lookup> = lookup_with.map(|v| v.to_str().unwrap().parse().unwrap());
 
  lookup_with.unwrap_or_else(|| match target_os.as_str() {
- // users have reported that this is required for freebsd. I have not tested it.
- "freebsd" => Lookup::Link,
+ // Users have reported that this is required for freebsd and illumos. Not tested by
+ // Cody P Schafer.
+ "freebsd" | "illumos" => Lookup::Link,
 
  // openzfs on osx has the `libzfs_core.pc` file, installed into
  // `/usr/local/zfs/lib/pkgconfig`. Users _must_ ensure this is part of their