before this change, we assume that scylla's CMake script includes
Seastar's CMake script.
but we are going to consume Seastar using its .pc files or its CMake
config files instead of including it directly. more over these helper
functions are not part of Seastar's public interface.
actually the same applies to the `check_headers()` helper, which was
adapted from seastar's CheckHeaders.cmake.
so to be prepared for this change, let's define these generate helper
functions in scylla.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
b2dc261841