Generate recursive meta language dependencies.

Cargo doesn't scan a directory for changed dependencies recursively, so do that
as part of the build.py script.

meta/gen_build_deps.py

@@ -0,0 +1,36 @@
+"""
+Generate build dependencies for Cargo.
+
+The `build.py` script is invoked by cargo when building libcretonne to
+generate Rust code from the instruction descriptions. Cargo needs to know when
+it is necessary to rerun the build script.
+
+If the build script outputs lines of the form:
+
+    cargo:rerun-if-changed=/path/to/file
+
+cargo will rerun the build script when those files have changed since the last
+build.
+"""
+
+import os
+from os.path import dirname, abspath, join
+
+
+def source_files(top):
+    """
+    Recursively find all interesting source files and directories in the
+    directory tree starting at top. Yield a path to each file.
+    """
+    for (dirpath, dirnames, filenames) in os.walk(top):
+        yield dirpath
+        for f in filenames:
+            if f.endswith('.py'):
+                yield join(dirpath, f)
+
+
+def generate():
+    print "Dependencies from meta language directory:"
+    meta = dirname(abspath(__file__))
+    for path in source_files(meta):
+        print "cargo:rerun-if-changed=" + path