This moves the generated files out of the source tree and into
the build directory. They are now regenerated each time they are
needed, based on the timestamp of the generated file compared to
the timestamp of the lemon file.
To do this, we also bundle lemon into the thirdparty directory
and build it for ourselves since it is a very tiny program and
not all platforms seem to distribute it in a consistent manner.
Fixes https://gitlab.com/kicad/code/kicad/issues/5013
Ian McInerney