Log Message:
lowdown: update to 0.6.4.

pkgsrc changes:

- Fix LICENSE marker, lowdown is ISC licensed
- Enable test suite
- Remove forgotten checksum for patch-Makefile in distinfo

ChangeLog:

Version 0.6.4, 2020-03-29

- Enable and enforce portability by also running regression tests over
  all operating systems and architectures. Updates oconfigure.