Create happy_birthday_posix.c

A parallel thread, happy birthday card, with two posit threads and very-fast, global transfer of variables between threads and main.

Needs pthreads installed: i.e  sudo apt-get install libpthread-stubs0-dev

After exection
  type echo $?

at the command line after execution
after
1. Change line 21 for different people.
2. Change line 47 as appropriate