Update to upstream version 2.18.12

Detailed maintainer log:

• Update to 2.18.12
• Add new dependency textproc/xerces-c3
• Swap position of USES and USE_GITHUB
• USES=sqlite:3 instead sqlite
• Use GRASS7 now, that it is in the ports
• No direct deps to PostgreSQL and PostGIS
• Update pkg-plist

PR: 221644
Submitted by: rhurlin@gwdg.de (maintainer)