agent, tests: Support --disable-scdaemon build case.

* agent/command.c (cmd_scd): Support !BUILD_WITH_SCDAEMON.
* tests/openpgp/defs.scm (create-gpghome): Likewise.
* tests/gpgsm/gpgsm-defs.scm (create-gpgsmhome): Likewise.

We could modify gpg-agent to remove all support of scdaemon, with no
inclusion of call-scd.c, divert-scd.c, and learncard.c, but it would
not be worth to do that.

• GnuPG-bug-id: T3316
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>