devel/libsigc++20: Update to 2.12.1
- Add GitHub mirror
- Update WWW entry
- Remove INSTALL_TARGET (unused)
Changelog:
https://github.com/libsigcplusplus/libsigcplusplus/releases/tag/2.12.1
PR: 280173
Approved by: gnome (fluffy) via Matrix
Sponsored by: Blinkinblox