cmake: Fix race condition on creating docbook based files

With the previous implementation, race conditions could arise because a generated intermediate file was used by multiple targets.

To fix the mentioned problem, the macro 'generate_docbook_file' has been integrated into a in a new macro 'add_docbook' to simplify the dependency chain and make it easier to use.

When using an xml template with the 'TEMPLATE' parameter, a separate intermediate xml file is used for each generated output file to avoid overwriting each other, which was the main cause of the described problem.

Due to the adaptation of the calling conventions it was necessary to introduce the parameter 'MAN_CATEGORY'.

Fixes #381 (closed)

Signed-off-by: Ralf Habacker ralf.habacker@freenet.de