Implement wasm trap handlers. (#27)

* Implement wasm trap handlers.

This adds signal handlers based on SpiderMonkey's signal-handler code.
The functionality for looking up the trap code and wasm bytecode offset
isn't yet implemented, but this is a start.

I considered rewriting this code in Rust, but decided against it for now
as C++ allows us to talk to the relevant OS APIs more directly.

Fixes #15.

* Compile with -std=c++11.

* Refactor InstallState initialization.

* Compile with -fPIC.

* Factor out the code for calling a wasm function with a given index.

* Fix unclear wording in a comment.

lib/execute/build.rs

@@ -0,0 +1,38 @@
+extern crate bindgen;
+extern crate cmake;
+extern crate regex;
+
+use cmake::Config;
+use regex::Regex;
+use std::env;
+use std::path::PathBuf;
+
+fn main() {
+    let dst = Config::new("signalhandlers").build();
+
+    println!("cargo:rustc-link-search=native={}", dst.display());
+    println!("cargo:rustc-link-lib=static=SignalHandlers");
+
+    let mut bindings_builder = bindgen::Builder::default()
+        .header("signalhandlers/SignalHandlers.h")
+        .whitelist_type("CodeSegment")
+        .whitelist_type("TrapContext")
+        .whitelist_type("jmp_buf")
+        .whitelist_function("EnsureEagerSignalHandlers");
+
+    // If we're compiling for Darwin, compile in extra Darwin support routines.
+    if Regex::new(r"-darwin[[:digit:].]*$")
+        .unwrap()
+        .is_match(&env::var("TARGET").unwrap())
+    {
+        bindings_builder = bindings_builder.whitelist_function("EnsureDarwinMachPorts");
+    }
+
+    let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());
+
+    bindings_builder
+        .generate()
+        .expect("Unable to generate bindings")
+        .write_to_file(out_path.join("signalhandlers.rs"))
+        .expect("Couldn't write bindings!");
+}