math/saga: Update to 8.4.0

Some of the visible changes are:
- Complete change from bitmap based to svg buttons, thanks to wxgtk32
- Show coordinates in 3D views (via 'l' key)
- Use CMake's 'FindGDAL.cmake'
- Overhaul of the about dialog
- Update ClipperLib to version 2
- Update help link to point to the new help page in the wiki
- and many more fixes and new features ...

GIT_TAGNAME is needed for the about dialog.

Changelog: https://sourceforge.net/p/saga-gis/wiki/Changelog%208.4.0/attachment/changelog_saga_8.4.0.txt

Reported by:	portscout, Repology