This adds a CMake option (DOLPHIN_DEFAULT_UPDATE_TRACK) to allow
configuring SCM_UPDATE_TRACK_STR. This is needed to enable auto-updates
in Windows CMake builds by default.
The built-in `configure_file` command correctly handles the case where
none of the variables change and scmrev.h doesn't need to be rebuilt.
This saves a full re-link of Dolphin any time CMake is re-run.
