Provide the C WASI implementation as an option.

This adds the C WASI implementation as a new crate, wasmtime-wasi-c,
and adds a command-line flag to the wasmtime command-line driver to
select which WASI implementation to use.

wasmtime-wasi-c/build.rs

@@ -0,0 +1,36 @@
+extern crate bindgen;
+extern crate cmake;
+
+use cmake::Config;
+use std::env;
+use std::path::PathBuf;
+
+fn main() {
+    let dst = Config::new("sandboxed-system-primitives").build();
+
+    println!("cargo:rustc-link-search=native={}", dst.display());
+    println!("cargo:rustc-link-lib=static=SandboxedSystemPrimitives");
+
+    let bindings_builder = bindgen::Builder::default()
+        .header("sandboxed-system-primitives/include/wasmtime_ssp.h")
+        .header("sandboxed-system-primitives/src/posix.h")
+        .whitelist_function("wasmtime_ssp_.*")
+        .whitelist_function("fd_table_init")
+        .whitelist_function("fd_table_insert_existing")
+        .whitelist_function("fd_prestats_init")
+        .whitelist_function("fd_prestats_insert")
+        .whitelist_function("argv_environ_init")
+        .whitelist_type("__wasi_.*")
+        .whitelist_type("fd_table")
+        .whitelist_type("fd_prestats")
+        .whitelist_type("argv_environ_values")
+        .whitelist_var("__WASI_.*");
+
+    let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());
+
+    bindings_builder
+        .generate()
+        .expect("Unable to generate bindings")
+        .write_to_file(out_path.join("wasmtime_ssp.rs"))
+        .expect("Couldn't write bindings!");
+}