Use builtin reader/writer lock on Windows

Several reader writer functions on Windows such as [TryAcquireSRWLockShared](http://msdn.microsoft.com/en-us/library/windows/desktop/dd405524(v=vs.85).aspx) are only available on Windows 7/2008R2 or later. To support Windows Vista/2008 and before, I am switching the Windows build to use the new builtin reader/writer lock.

Build: Mac OS 10.9, Windows 8.1, Ran s_all