Make flex/bison checks stricter in Git trees

When running ./configure on a system without Flex/Bison it’s easy to miss the
warning that flies past and then run into compilation error instead. When it
happened to a colleague yesterday a brief discussion came to the conclusion
that it would be neat it the flex and bison checks took the existence of the
generated files into consideration.

Attached patch scans for the generated files and iff flex or bison isn’t found
in a non-cross compilation build, errors out in case the generated filed don’t
exist while retaining the warning in case they do. This maintains the current
warning message for downloaded distribution trees while it make the check
strict on Git trees. It does add a hardcoded list of files which it can be
argued how nice that is even though the list rarely change (there might be a
cleaner way but I couldn’t see one). Also includes a tiny copy-edit fix on the
flex warning to make it match the bison one since that seemed better.

If this is at all of interest I can add to the commitfest.

cheers ./daniel