Fix for Linux

[mominul]

Hi, this pull request fixes errors that are occurring when compiling in Linux

• On Linux the callback function argument is UnsafeMutableRawPointer?
• pthread_t is non optional on Linux

Thanks to @jakeheis for his fork

I haven't tested it on macOS

[esttorhe]

I know that this repo is not mine or anything; but as seeing that the code inside the Linux path doesn't contain stuff that would fail compiling on macOS; wouldn't it be better to have something like macOSCallback and linuxCallback and then only applying the filtering inside of callback:

func callback(x: UnsafeMutableRawPointer?) -> UnsafeMutableRawPointer? {
#if(OSX)
   return macOSCallback(x)
#else
   return linuxCallback(x)
#endif
}

Edit: I think this would make it more readable; happy to discuss if this is not true or doesn't make sense or even if it really doesn't add any value.