message: Catch OSError in gen_git_source.py subprocess call.
  OSError occurs if git cannot be found. This is a initial fix for some
  Windows build errors.
diff: @@ -125,7 +125,7 @@ def configure(src_base_path, gen_path, debug=False):
  try:
  # In python 3.5, symlink function exists even on Windows. But requires
  # Windows Admin privileges, otherwise an OSError will be thrown.
  - if hasattr(os, 'symlink'):
  + if hasattr(os, "symlink"):
  os.symlink(src, os.path.join(gen_path, target))
  else:
  shutil.copy2(src, os.path.join(gen_path, target))
  @@ -162,7 +162,7 @@ def get_git_version(git_base_path):
  str("--work-tree=" + git_base_path), "describe", "--long", "--tags"
  ]).strip())
  return val if val else unknown_label
  - except subprocess.CalledProcessError:
  + except (subprocess.CalledProcessError, OSError):
  return unknown_label
  