Patched for `cannot convert pid (type _Ctype_pthread_t) to type uintpt` error

```
../go-pthreads/Thread.go:59[/var/folders/vt/c95fccvn5lg337mcd60h4bsw0000gn/T/go-build509599842/github.com/spikeekips/go-pthreads/_obj/Thread.cgo1.go:49]: cannot convert pid (type _Ctype_pthread_t) to type uintptr
../go-pthreads/Thread.go:77[/var/folders/vt/c95fccvn5lg337mcd60h4bsw0000gn/T/go-build509599842/github.com/spikeekips/go-pthreads/_obj/Thread.cgo1.go:70]: cannot convert t (type Thread) to type _Ctype_pthread_t
```

Tested in OSX, golang 1.3.1 gccgo, based on gcc49.