Refactor `exec_pyg_scripts()` so that it is usable externally

via a new ``pyg2hpp.py`` script. This will allow other build systems to
take ``pyg`` files and generate the corresponding ``hpp`` files.

scripts/pyg2hpp.py

@@ -0,0 +1,42 @@
+#!/usr/bin/env python
+"""
+Reads a pyg file and emits the corresponding
+C++ header file into the specified destination
+directory.
+"""
+import mk_util
+import argparse
+import logging
+import os
+import sys
+
+def main(args):
+  logging.basicConfig(level=logging.INFO)
+  parser = argparse.ArgumentParser(description=__doc__)
+  parser.add_argument("pyg_file", help="pyg file")
+  parser.add_argument("destination_dir", help="destination directory")
+  pargs = parser.parse_args(args)
+
+  if not os.path.exists(pargs.pyg_file):
+    logging.error('"{}" does not exist'.format(pargs.pyg_file))
+    return 1
+
+  if not os.path.exists(pargs.destination_dir):
+    logging.error('"{}" does not exist'.format(pargs.destination_dir))
+    return 1
+
+  if not os.path.isdir(pargs.destination_dir):
+    logging.error('"{}" is not a directory'.format(pargs.destination_dir))
+    return 1
+
+  pyg_full_path = os.path.abspath(pargs.pyg_file)
+  destination_dir_full_path = os.path.abspath(pargs.destination_dir)
+  logging.info('Using {}'.format(pyg_full_path))
+  # Use the existing infrastructure to do this
+  mk_util.CURRENT_PYG_HPP_DEST_DIR = destination_dir_full_path
+  mk_util._execfile(pyg_full_path, mk_util.PYG_GLOBALS)
+  return 0
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv[1:]))
+