Add vendored LinuxCNC sources

[jamwaffles]

The docs.rs build fails because the build script expects a location to an external LinuxCNC source checkout.

So:

1. Let's add a vendored-linuxcnc (or better named) feature to allow usage of a bundled version of the LinuxCNC sources. This should be pinned to the latest stable LinuxCNC release.
2. Add another feature for internal use which only generates bindings and does not attempt to build anything against the LinuxCNC sources. This should really be used for docs.rs builds only.