Disable creation of .pyc files.

Cretonne's python scripts aren't run very often, so there's little
benefit in creating .pyc files. And the .pyc files cause trouble for
some vendoring scripts. So disable them.

lib/cretonne/build.rs

@@ -59,8 +59,11 @@ fn main() {
     let build_script = meta_dir.join("build.py");
 
     // Launch build script with Python. We'll just find python in the path.
+    // Use -B to disable .pyc files, because they cause trouble for vendoring
+    // scripts, and this is a build step that isn't run very often anyway.
     let status = process::Command::new("python")
         .current_dir(crate_dir)
+        .arg("-B")
         .arg(build_script)
         .arg("--out-dir")
         .arg(out_dir)