title: ssize_t is undefined
body: <p>In my code I'm using ssize_t but when I try to compile project I got 
  error: unknown type name ‘ssize_t’; did you mean ‘size_t’?</p>
  
  <p>To compile project I'm using cc -std=c11 -O3 
  I'm including stdint.h and I've also tried stddef.h and others.</p>
  
  <p>Do I have to use some flags or what?</p>