math/saga: Update to 7.9.1

This minor release is expected to be the last version before 8.0.0.

• Several fixes in tools and toolchains
• Reimplement detection of PDAL and build of io_pdal
• Enable build of imagery_isocluster tool
• Updated link to Proj georeference and public source

Changelog: https://sourceforge.net/p/saga-gis/wiki/Changelog%207.9.1/

Reported by: Repology