Allow 'source'ing the same file from multiple places (eg. from
different files, and/or under different conditions).

To avoid circular inclusion, scan the source-ancestry of the
current file, and abort if already sourced in this branch.

Regenerate the pre-parsed lex.zconf.c_shipped file.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>