expat picking up dockbook2man and erroring when XML support is not enabled.

Summary

Found by ita on Discord

• https://discord.com/channels/820452222382112799/1257160543580590141/1300950748561018911

By following these steps:

• https://docs.rtems.org/branches/master/user/hosts/posix.html#xubuntu

Steps to reproduce

Pre-set options