gpg: Let --debug clock time sign and verify.

* configure.ac (ENABLE_LOG_CLOCK): New ac_define and option.
* common/logging.c (log_clock): Use ENABLE_LOG_CLOCK to enable
timestamp printing.
* g10/call-agent.c (agent_pksign): Time signing.
* g10/sig-check.c (check_signature_end_simple): Time verification.

Timing for verification is limited to data signatures because this is
the most common thing to evaluate. We should consider to change
log_clock to printf style so that we could print the signature class
and other info.

• Signed-off-by: Werner Koch <wk@gnupg.org>