Sphinx build always rebuilds docs even when it's not necessary

[ezyang]

[ezyang]

It's probably because of this:

for i in range(len(sphinx_gallery_conf['examples_dirs'])):
    gallery_dir = sphinx_gallery_conf['gallery_dirs'][i]
    source_dir = sphinx_gallery_conf['examples_dirs'][i]
    # Create gallery dirs if it doesn't exist
    try:
        os.mkdir(gallery_dir)
    except OSError:
        pass

    # Copy rst files from source dir to gallery dir
    for f in glob.glob(os.path.join(source_dir, '*.rst')):
        shutil.copy(f, gallery_dir)