Add scaffolding for a Python build script.

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

meta/build.py

@@ -0,0 +1,11 @@
+# Second-level build script.
+#
+# This script is run from src/libcretonne/build.rs to generate Rust files.
+
+import argparse
+
+parser = argparse.ArgumentParser(description='Generate sources for Cretonne.')
+parser.add_argument('--out-dir', help='set output directory')
+
+args = parser.parse_args()
+out_dir = args.out_dir