instead of checking the availability of a required program, let's
use the `REQUIRED` argument introduced by CMake 3.18, simpler this
way.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#15447
as it builds headers like "gen/Cassandra.h", and the target
uses "interface" via these headers, so "interface" is obliged
to expose this include directory.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>