Log Message:
libcaca: Update to 0.99.20

Special notes:
- building without imlib2 is broken upstream, remove the option.
- upstream switched to github.
- verified to build on freebsd, netbsd, linux, macos by drecklypkg CI.

Upstream changes:
- IPv6 support in cacaserver
- fixed a bug from 2004 that caused PDF documentation generation to
  fail
- memory allocation functions are now more robust
- numerous fixes for memory leaks and invalid memory accesses:
    CVE-2021-30498
    CVE-2021-30499
    CVE-2021-3410
    CVE-2018-20546
    CVE-2018-20547
    CVE-2018-20545
    CVE-2018-20548
    CVE-2018-20549