in fabab2f4, we introduced preemption_source, and added
`SCYLLA_ENABLE_PREEMPTION_SOURCE` preprocessor macro to enable
opt-in the pluggable preemption check.
but CMake building system was not updated accordingly.
so, in this change, let's sync the CMake building system with
`configure.py`.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#19951