test: Ensure defined macro without a value is None
test: Ensure defined macro without a value is None

According to the `distutils.core.Extension` documentation, if a macro is
defined without a value, it should be set to `None`.  This allows for
the following pattern in `setup.py`:

    setuptools.Extension(
        # ...
        **pkgconfig.parse('...')
    )

Signed-off-by: Eric N. Vander Weele <4c4001a597f85b23ba16d5c26e44733bbe755332@gmail.com>

matze/pkgconfig