Add some initial support for the logging framework that should be available in
Serf 1.4.0 and later.

This should allow getting some logging output on stderr with:
$ svn --config-option servers:global:serf-log-components=255 \
      ls https://svn.apache.org/repos/asf/subversion

And all output with
$ svn --config-option servers:global:serf-log-components=255 \
      --config-option servers:global:serf-log-level=8 \
      ls https://svn.apache.org/repos/asf/subversion

Currently you need to compile against serf trunk to enable this.

* notes/knobs
  Add SVN_SERF_NO_LOGGING.

* subversion/include/svn_config.h
  (SVN_CONFIG_OPTION_SERF_LOG_COMPONENTS,
   SVN_CONFIG_OPTION_SERF_LOG_LEVEL): New define.

* subversion/libsvn_ra_serf/serf.c
  (load_config): When serf >= 1.4 load serf log configuration.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541561 13f79535-47bb-0310-9956-ffa450edef68