FindHUNSPELL.cmake make hunspell soft-mandatory

See karchive!60 (merged)
See https://conf.kde.org/event/5/contributions/136/attachments/80/96/flatpak_and_kde.pdf
page 11
By default if I build Sonnet using kdesrc-build Qt6, Sonnet will build
even if there are zero spell checking backends installed.
"-- The following OPTIONAL packages have not been found:

ASPELL, Spell checking support via Aspell, http://aspell.net/
HSPELL, Spell checking support for Hebrew, http://ivrix.org.il/projects/spell-checker/
HUNSPELL, Spell checking support via Hunspell, https://hunspell.github.io/
VOIKKO, Spell checking support via Voikko, https://voikko.puimula.org/"

kdesrc-run kanagram fails with error
"kf.sonnet.core: Sonnet: No speller backends available!"