Use the gpgrt log functions if possible.

* common/logging.c: Do not build any code if we can use the gpgrt_log
functions.
(log_logv_with_prefix): Rename to log_logv_prefix and change order of
args so that this function matches its printf like counterpart
gpgrt_logv_prefix.  Change all callers.
(log_debug_with_string): Rename to log_debug_string. Change all
callers.
(log_printhex): Move first arg to end so that this function matches
its printf like counterpart gpgrt_log_printhex.  Change all callers.
* common/logging.h: Divert to gpgrt/libgpg-error if we can use the
gpgrt_log functions.
(bug_at): Add inline versions if we can use the gpgrt_log functions.
* configure.ac (GPGRT_ENABLE_LOG_MACROS): Add to AH_BOTTOM.
(mycflags): Add -Wno-format-zero-length.
--

This patch enables the use of the log function from libgpgrt (aka
libgpg-error).  Instead of checking a version number, we enable them
depending on macros set by recent gpg-error versions.  Eventually the
whole divert stuff can be removed.

The -Wno-format-zero-length is required because log_printhex can be
called with an empty format string.  Note that this is fully specified
standard C behaviour.

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

sm/certdump.c

@@ -167,7 +167,7 @@ gpgsm_dump_string (const char *string)
       else
         {
           log_printf ( "[ ");
-          log_printhex (NULL, string, strlen (string));
+          log_printhex (string, strlen (string), NULL);
           log_printf ( " ]");
         }
     }