bindings: rust: build against pkg-config info

bindings: rust: build against pkg-config info

This change replaces building against "bundled" headers by always
building agains system headers (while following standard conventions to
allow users to specify the version to build against).

Reasoning:

Previously, the code generated the bindings based on the headers, but
then links against `-lgpiod` without further specifying where that is
coming from.

This results in some challenges and problems:

1. Packaging a Rust crate with `cargo package` requires the folder
   containing the Cargo.toml to be self-contained. Essentially, a tar
   ball with all the sources of that folder is created. Building against
   that tar ball fails, since the headers files passed to bindgen are
   a relative path pointing outside of that folder.

2. While, for example, the cxx bindings are built AND linked against
   the build results, the packaging situation for C++ libraries is a
   bit different compared to Rust libs. The C++ libs will likely get
   built as part of the larger libgpiod build and published together
   with the C variant.

   In Rust, the vast majority of people will want to build the glue-code
   during the compilation of the applications that consume this lib.

   This may lead to inconsistencies between the bundled headers and the
   libraries shipped by the user's distro. While ABI should hopefully
   be forward-compatible within the same MAJOR number of the .so,
   using too new headers will likely quickly lead to mismatches with
   symbols defined in the lib.

3. Trying to build the core lib as part of the Rust build quickly runs
   into similar packaging issues as the existing solution. The source
   code of the C lib would need to become part of some package
   (often people opt to pull it in as a submodule under their -sys crate
   or even create a separate -src package [1]). This clearly does not
   work well with the current setup...

Since building against system libs is probably? what 90%+ of the people
want, this change hopefully addresses the problems above. The
system-deps dependency honors pkg-config conventions, but also allows
users flexible ways to override the defaults [2]. Overall, this keeps
things simple while still allowing maximum flexibility.

Since the pkg-config interface is just telling us which include paths to
use, we switch back to a wrapper.h file that includes the real gpiod.h.

Once Rust bindings require a lower version floor, the version metadata
can also be updated to help telling users that their system library is
too old.

In order to support people hacking on the Rust bindings, building with
make will automatically set the right set of environment variables.
In case people want to customize it when building without `make`, a
reference to the system_deps documentation is given in the README.md.

[1] https://github.com/alexcrichton/openssl-src-rs
[2] https://docs.rs/system-deps/latest/system_deps/#overriding-build-flags

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>