editors/emacs: Update to version 26.3
Port changes:
- Remove JSON knob, because Emacs has no configurable json option in versions prior to and including 26.3. The knob should not have been added in r472261.
- Add USES+=gnome unless flavor is nox. Do this because defining INSTALLS_ICONS adds USE_GNOME+=gtk-update-icon-cache.
Upstream changes:
https://www.gnu.org/software/emacs/news/NEWS.26.3
Submitted by: HIROSE Yuuji <yuuji@gentei.org> [canna patch updates]
Differential Revision: https://reviews.freebsd.org/D21193