Most plugins do not have a 'headers' directory.
Avoids warnings from frama-c-hdrck when giving such non-existing directories
to the -header-dirs option.

This is less efficient but circumvents a bug in tar --concatenate
(https://superuser.com/questions/941475/concatenate-multiple-tar-files-in-one-command)

- check headers before changing them
- fail to build open-source distrib with close source file