FUNCTION 'PTHREAD_INIT' 

The problem was that compilation would fail with a warning:
Implicit declaration of function 'pthread_init' if MySQL was
compiled on OS X 10.7 (Lion). The reason was that pthread_init()
is now part of an internal OS X pthread library so it was found
by CMake.

This patch fixes the problem by removing HAVE_PTHREAD_INIT and
related code. pthread_init() was specific to MIT-pthreads which
has not been supported since 4.1 and was therefore no longer
relevant.

No test case added.