def compat_system(source_dir):
    """
    Given a source dir, attempt to get a build system backend
    and requirements from pyproject.toml. Fallback to
    setuptools but only if the file was not found or a build
    system was not indicated.
    """
    try:
        system = load_system(source_dir)
    except (FileNotFoundError, KeyError):
        system = {}
    system.setdefault(
        'build-backend',
        'setuptools.build_meta:__legacy__',
    )
    system.setdefault('requires', ['setuptools', 'wheel'])
    return system

    Given a source dir, attempt to get a build system backend
    and requirements from pyproject.toml. Fallback to
    setuptools but only if the file was not found or a build
    system was not indicated.
    
product/gradle-plugin/src/main/python/pip/_vendor/pep517/build.py