When targeting Windows 8, the always_has_native_wait_notify static constant
would be set to false even though the native implementation of waiting
and notifying operations through WaitOnAddress would always be used.
To fix this, moved direct WaitOnAddress & co. usage to public header. This
also requires users to link with synchronization.lib, which is done
automatically on compilers that support auto-linking.
The compiled library still implements runtime detection of WaitOnAddress,
even when building for Windows 8 and later. This allows user's code to
be later compiled for Windows 7 and older and still work. This code is
not used when user's code targets Windows 8 or later.
As part of this refactoring, renamed has_synchronization test and
related CMake variable to better communicate that we're testing for
a library.
