This basically reverts a29fca70,
except that we are using python instead of make-inlined shell code to
do the work.

Reason: vcs_version.h needs to be up-to-date under all circumstances,
otherwise we will end up with wrong version information in binary
builds, which would divert developer resources into unproductive
confusion compensation.

With an all-in-one generate.py, we basically have the choice between:

- running it with each build, which breaks incremental builds as
  generate.py rewrites central includes like vcl.h

- adding version information to all generate.py-build files or similar
  mechanisms to avoid re-writing them unless they have actually
  changed.

- contradicting the argument given above

I think that, unless there are strong reasons for a single
generate.py, avoiding these issues by splitting functions is the best
option.