Search gpg-connect-agent in PATH at runtime.
* configure.ac: Do not hard-code full path to gpg-connect-agent.
By default, Scute should use the gpg-connect-agent program
present in PATH at runtime, unless the user chooses otherwise
at configure-time.
- GnuPG-bug-id: T3274
- Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>