gnucash-docs maint: Ensure our documentation is also found when gnucash is run in KDE

[Geert Janssens]

Updated	 via  https://github.com/Gnucash/gnucash-docs/commit/51374741 (commit)
	from  https://github.com/Gnucash/gnucash-docs/commit/cb0c0a41 (commit)



commit 513747413efc4732f5ae824ccc84316169b1523f
Author: Geert Janssens <geert at kobaltwit.be>
Date:   Thu Oct 20 11:25:19 2022 +0200

    Ensure our documentation is also found when gnucash is run in KDE
    
    Turns out KDE is not adhering to the xdg spec after all.
    It has support for the old ghelp mechanism, but when using
    the help: prefix, it falls back to an internal protocol.
    In that case the requested document is searched for in
    <prefix>/share/doc/HTML/<lang>.
    Extra caveat - the English documentation is searched for
    in a directory 'en' instead of 'C'.
    This is not a problem in yelp, only in khelpcenter. Unfortunately
    khelpcenter is the default application to process help: prefixes
    in a Kde plasma desktop, hence the workaround.



Summary of changes:
 cmake/AddXdgHelpTarget.cmake | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)