bindings: python: optionally include module in sdist

bindings: python: optionally include module in sdist

Optionally vendor libgpiod source into sdist so that the Python module can
be built from source, even with a missing or mismatched system libgpiod.

Add two new environment variables "LINK_SYSTEM_LIBGPIOD" and
"LIBGPIOD_VERSION" to control what kind of package setup.py will build.

In order to build an sdist or wheel package with a vendored libgpiod a
version must be specified via the "LIBGPIOD_VERSION" environment variable.

This will instruct setup.py to verify the given version against the list
in sha256sums.asc and ensure it meets or exceeds a LIBGPIOD_MINIMUM_VERSION
required for compatibility with the bindings.

It will then fetch the tarball matching the requested version from
mirrors.edge.kernel.org, verify the sha256 signature, unpack it, and copy
the lib and include directories into the package root so they can be
included in sdist or used to build a binary wheel.

eg: LIBGPIOD_VERSION=2.1.0 python3 setup.py sdist

Will build a source distribution with gpiod version 2.1.0 source included.

It will also save the gpiod version into "libgpiod-version.txt" so that it
can be passed to the build when the sdist is built by pip.

Requiring an explicit version ensures that the Python bindings - which
can be changed and versions independent of libgpiod - are built against a
stable libgpiod release.

In order to force a package with vendored gpiod source to link the system
libgpiod, the "LINK_SYSTEM_LIBGPIOD" environment variable can be used:

eg: LINK_SYSTEM_LIBGPIOD=1 pip install libgpiod

Signed-off-by: Phil Howard <phil@gadgetoid.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>