Add gcr as an optional depency

Description:

Currently pinentry-gnome3 breaks if gcr is not installed and there's no indication that gcr is needed instead of gcr-4 (at least for the gnome3 frontend) in the package page

Additional info:

  • package version(s): 1.3.1-2
  • config and/or log files:
  • link to upstream bug report, if any:

Steps to reproduce:

  1. Install pinentry regularly without installing gcr
  2. Try signing a git commit without setting a different pinentry-program in gpg-agent.conf