Could this be replaced with

#ifdef WIN32
  typedef long v8ThreadId;
#else
  typedef pthread_t v8ThreadId;
#endif

and change the usages to use the new type v8ThreadId?

Good catch, and great solution, although I don't believe xlc++ can handle the edit. I get the following error on compilation:

invalid operands to binary expression
('threadId' (aka 'pthread_t') and 'threadId')
if((isWIN32 && (myThreadId == v8ThreadId)) || (!isWIN32 && pthread_equal(myThreadId, v8ThreadId))) {

This is being thrown on:
myThreadId == v8ThreadId
even though it should have short circuited on the isWIN32 check

I was hoping that by using the typedef you could get rid of a lot of the edits below. For example my_getThreadId could just be written:

v8ThreadId my_getThreadId() {		
  #ifdef WIN32
    return GetCurrentThreadId();
  #else
    return pthread_self();
  #endif
}

then we should be able to get rid of the isWIN32 logic all together.

If you want to use pthread_equal could #define v8ThreadIdEquals(a,b) ...

I went ahead and took the logic out of the condition and put it into a function v8ThreadIdEquals instead. Everything is working perfectly for me on z/OS and OSX