Merge r1917578 from trunk:

Merge r1917578 from trunk:

On Linux use the real thread id via gettid() in error logging,
where available, rather than the (meaningless) default
pthread_self()-as-integer interpretation:

* configure.in: Define DEFAULT_LOG_TID on Linux if gettid() is available.

* server/log.c: Define DEFAULT_LOG_TID as NULL by default.
  (do_errorlog_default): Use DEFAULT_LOG_TID as the argument to log_tid().

Reviewed by: jorton, ylavic, icing

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1918489 13f79535-47bb-0310-9956-ffa450edef68