Wrap build.py's contents in a main function.

This prevents its local variables from becoming global variables.

lib/cretonne/meta/build.py

@@ -14,7 +14,11 @@ import gen_legalizer
 import gen_registers
 import gen_binemit
 
-parser = argparse.ArgumentParser(description='Generate sources for Cretonne.')
+
+def main():
+    # type: () -> None
+    parser = argparse.ArgumentParser(
+            description='Generate sources for Cretonne.')
     parser.add_argument('--out-dir', help='set output directory')
 
     args = parser.parse_args()
@@ -30,3 +34,7 @@ gen_legalizer.generate(isas, out_dir)
     gen_registers.generate(isas, out_dir)
     gen_binemit.generate(isas, out_dir)
     gen_build_deps.generate()
+
+
+if __name__ == "__main__":
+    main()