Add a crate to interface with the WebAssembly spec interpreter

The WebAssembly spec interpreter is written in OCaml and the new crate
uses `ocaml-interop` along with a small OCaml wrapper to interpret Wasm
modules in-process. The build process for this crate is currently
Linux-specific: it requires several OCaml packages (e.g. `apt install -y
ocaml-nox ocamlbuild`) as well as `make`, `cp`, and `ar`.

crates/fuzzing/wasm-spec-interpreter/build.rs

@@ -0,0 +1,54 @@
+/// Build the OCaml code and statically link it into the Rust library; see the
+/// [ocaml-interop
+/// example](https://github.com/tezedge/ocaml-interop/blob/master/testing/rust-caller/build.rs)
+/// for more details. After playing with this a bit, I discovered that the best
+/// approach to avoid missing symbols was to imitate `dune`: I observed `rm -rf
+/// _build && dune build ./ocaml/interpret.exe.o --display=verbose` and used
+/// that as a pattern, now encoded in `ocaml/Makefile` for easier debugging.
+use std::{env, process::Command};
+
+const LIB_NAME: &'static str = "interpret";
+const OCAML_DIR: &'static str = "ocaml";
+
+fn main() {
+    if cfg!(feature = "build-libinterpret") {
+        build();
+    }
+}
+
+fn build() {
+    let out_dir = &env::var("OUT_DIR").unwrap();
+
+    // Re-run if changed.
+    println!("cargo:rerun-if-changed={}/{}.ml", OCAML_DIR, LIB_NAME);
+    println!("cargo:rerun-if-changed={}/Makefile", OCAML_DIR);
+
+    if let Some(other_dir) = env::var_os("FFI_LIB_DIR") {
+        // Link with a library provided in the `FFI_LIB_DIR`.
+        println!("cargo:rustc-link-search={}", other_dir.to_str().unwrap());
+        println!("cargo:rustc-link-lib=static={}", LIB_NAME);
+    } else {
+        // Build the library to link to.
+        build_lib(out_dir, OCAML_DIR);
+        println!("cargo:rustc-link-search={}", out_dir);
+        println!("cargo:rustc-link-lib=static={}", LIB_NAME);
+    }
+
+    // Enabling this feature alerts the compiler to use the `with_library`
+    // module.
+    println!("cargo:rustc-cfg=feature=\"has-libinterpret\"");
+}
+
+// Build the OCaml library into Cargo's `out` directory.
+fn build_lib(out_dir: &str, ocaml_dir: &str) {
+    let status = Command::new("make")
+        .arg(format!("BUILD_DIR={}", out_dir))
+        .current_dir(ocaml_dir)
+        .status()
+        .expect("Failed to execute 'make' command to build OCaml library");
+
+    assert!(
+        status.success(),
+        "Failed to build the OCaml library using 'make'."
+    )
+}