gettid doesn't compile for old glibc

ubuntu@VM-48-15-ubuntu:~$ ldd --version
ldd (Ubuntu EGLIBC 2.19-0ubuntu6.13) 2.19
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.

ubuntu@VM-48-15-ubuntu:~$ gcc -g -Wall -o test test.c
test.c: In function ‘main’:
test.c:7:2: warning: implicit declaration of function ‘gettid’ [-Wimplicit-function-declaration]
  pid_t a = gettid();
  ^
/tmp/ccxkbKZH.o: In function `main':
/home/ubuntu/test.c:7: undefined reference to `gettid'
collect2: error: ld returned 1 exit status