Add scaffolding for a Python build script.

Hook up a Cargo build script that runs a Python script in the meta directory.

src/libcretonne/Cargo.toml

@@ -2,6 +2,7 @@
 authors = ["The cwCretonneRust Project Developers"]
 name = "cretonne"
 version = "0.0.0"
+build = "build.rs"
 
 [lib]
 name = "cretonne"

src/libcretonne/build.rs

@@ -0,0 +1,49 @@
+
+// Build script.
+//
+// This program is run by Cargo when building libcretonne. It is used to generate Rust code from
+// the language definitions in the meta directory.
+//
+// Environment:
+//
+// OUT_DIR
+//     Directory where generated files should be placed.
+//
+// The build script expects to be run from the directory where this build.rs file lives. The
+// current directory is used to find the sources.
+
+
+use std::env;
+use std::process;
+
+fn main() {
+    let out_dir = env::var("OUT_DIR").expect("The OUT_DIR environment variable must be set");
+
+    println!("Build script generating files in {}", out_dir);
+
+    let mut cur_dir = env::current_dir().expect("Can't access current working directory");
+
+    // We're in src/libcretonne. Find the top-level directory.
+    assert!(cur_dir.pop(), "No parent 'src' directory");
+    assert!(cur_dir.pop(), "No top-level directory");
+    let top_dir = cur_dir.as_path();
+
+    // Scripts are in $top_dir/meta.
+    let meta_dir = top_dir.join("meta");
+    let build_script = meta_dir.join("build.py");
+
+    // Let Cargo known that this script should be rerun if anything changes in the meta directory.
+    println!("cargo:rerun-if-changed={}", meta_dir.display());
+
+    // Launch build script with Python. We'll just find python in the path.
+    let status = process::Command::new("python")
+                     .current_dir(top_dir)
+                     .arg(build_script)
+                     .arg("--out-dir")
+                     .arg(out_dir)
+                     .status()
+                     .expect("Failed to launch second-level build script");
+    if !status.success() {
+        process::exit(status.code().unwrap());
+    }
+}