[PATCH] Use pthread_self() instead of getpid() in Dprintf

Thank you, but I am reluctant to apply this patch because:
1. it doesn't work on Windows: pthread_self must be #defined to GetCurrentThreadId. I don't know about OSX.
2. pthread_t is not a pointer: it is an opaque structure only supposed to be compared by pthread_equal. See this and this. So just casting it to a void* doesn't seem portable.