gnucash-docs maint: Add option to enable/disable html install rules

[Geert Janssens]

Updated	 via  https://github.com/Gnucash/gnucash-docs/commit/bdd42fe5 (commit)
	from  https://github.com/Gnucash/gnucash-docs/commit/b3f46b4f (commit)



commit bdd42fe57aaff81b97f3868a31347e4ffc1091eb
Author: Geert Janssens <geert at kobaltwit.be>
Date:   Wed Jul 6 16:58:51 2022 +0200

    Add option to enable/disable html install rules
    
    It's only used on MacOS and interferes with the linux distribution builds.
    The option is enabled by default on MacOS and disabled on other platforms.



Summary of changes:
 CMakeLists.txt            | 7 +++++++
 cmake/AddHtmlTarget.cmake | 7 ++++---
 2 files changed, 11 insertions(+), 3 deletions(-)