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/build.py

@@ -5,6 +5,7 @@
 import argparse
 import target
 import gen_instr
+import gen_build_deps
 
 parser = argparse.ArgumentParser(description='Generate sources for Cretonne.')
 parser.add_argument('--out-dir', help='set output directory')
@@ -15,3 +16,4 @@ out_dir = args.out_dir
 targets = target.all_targets()
 
 gen_instr.generate(targets, out_dir)
+gen_build_deps.generate()