Check for and use gettid() directly if available; glibc 2.30 and later

Check for and use gettid() directly if available; glibc 2.30 and later
provides a wrapper for the system call:

* configure.in: Check for gettid() and define HAVE_SYS_GETTID if
  gettid() is only usable via syscall().

* server/log.c (log_tid): Use gettid() directly if available.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1879591 13f79535-47bb-0310-9956-ffa450edef68