hack our way around missing libcrypt in cross-compilation (#438)

build.rs

@@ -1,11 +1,47 @@
-fn main() {
+use std::env;
+use std::error::Error;
+
+type Output = Result<(), Box<dyn Error>>;
+
+fn main() -> Output {
     // this is wild
     #[cfg(target_os = "macos")]
     {
         println!("cargo:rustc-link-lib=framework=foundation");
         println!("cargo:rustc-link-lib=framework=security");
     }
 
-    #[cfg(target_os = "linux")]
-    println!("cargo:rustc-link-lib=dylib=crypt");
+    let target = env::var("TARGET").ok();
+    let host = env::var("HOST").ok();
+
+    // This is an enormous hack.  Normally on Linux, Ruby would use
+    // libcrypt functions libruby-static.a would contain references to
+    // such functions, and we would need to specify -lcrypt via the
+    // link line below.
+    //
+    // But on a vanilla cross-compilation setup on Ubuntu, libcrypt
+    // doesn't exist for cross-compilation.  Ruby is OK with that (its
+    // configure script will detect the absence of a linkable
+    // libcrypt) and therefore in such a situation we would not need
+    // to specify -lcrypt.  Ruby's rbconfig.rb file, generated by its
+    // configure script, will helpfully include information about the
+    // presence or absence of libcrypt on the system.
+    //
+    // But we can't access that Ruby configuration information here:
+    // we have no control over when the build script for this crate
+    // runs vs. the build script for librubyfmt, and it is the latter
+    // that configures Ruby and generates the very files that we would
+    // need to look at to make the determination of whether to specify
+    // -lcrypt.
+    //
+    // So instead we make an assumption: if our target and host are
+    // different, then we're running in a vanilla-ish Ubuntu
+    // environment where libcrypt wouldn't be available.  In a more
+    // full-featured cross-compilation setup, this assumption is
+    // invalid, but it works well enough for our purposes.
+    if cfg!(target_os = "linux") && target.zip(host).map(|(t, h)| t == h).unwrap_or(false) {
+        println!("cargo:rustc-link-lib=dylib=crypt");
+    }
+
+    Ok(())
 }