fuzz: improve wasm-spec-interpreter documentation, add library path (#4007)

The documentation for the `wasm-spec-interpreter` was not up-to-date,
causing some confusion on non-Ubuntu machines. This change adds the
correct dependencies to install and includes the `libgmp` path for
Fedora by default (i.e., `/lib64`).

crates/fuzzing/wasm-spec-interpreter/README.md

@@ -15,25 +15,23 @@ This crate only builds in an environment with:
 - `ocamlopt`, `ocamlbuild` (available with, e.g., `dnf install ocaml`)
 - Linux tools (e.g. `ar`); currently it is easiest to build the static
   libraries in a single environment but this could be fixed in the future (TODO)
-
-Remember to retrieve the Wasm spec submodule:
-
-```
-git clone ... --recursive
-```
+- `libgmp`, for the OCaml `zarith` package
+- `git` is used by `build.rs` to retrieve the repository containing the Wasm
+  spec interpreter; it is safe to completely remove `ocaml/spec` to get a new
+  copy
 
 ### Build
 
 ```
-RUSTFLAGS=--cfg=fuzzing cargo build
+cargo build --features build-libinterpret
 ```
 
 Use `FFI_LIB_DIR=path/to/lib/...` to specify a different location for the static
-library (this is mainly for debugging). If the `--cfg=fuzzing` configuration is
+library (this is mainly for debugging). If the `build-libinterpret` feature is
 not provided, this crate will build successfully but fail at runtime.
 
 ### Test
 
 ```
-RUSTFLAGS=--cfg=fuzzing cargo test
+cargo test --features build-libinterpret
 ```