gnucash-docs maint: Make calibre optional in cmake

[Frank H.Ellenberger]

Updated	 via  https://github.com/Gnucash/gnucash-docs/commit/6d2877c5 (commit)
	from  https://github.com/Gnucash/gnucash-docs/commit/ec170e33 (commit)



commit 6d2877c523c090d45f96b9fc02b884bd92d45f6c
Author: Frank H. Ellenberger <frank.h.ellenberger at gmail.com>
Date:   Sat Jun 19 04:10:15 2021 +0200

    Make calibre optional in cmake



Summary of changes:
 CMakeLists.txt | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)