build: persist Seastar options in SeastarConfig.cmake

SeastarConfig.cmake is installed/redistributed by Seastar. this
file is consumed by `find_package(Seastar)`. but the options configuring
the build dependencies should be consistent when build Seastar and when
consuming the built Seastar libraries. for instance, when building
Seastar, a feature relying on a certain package is enabled. the
Seastar application consuming the prebuild Seastar should check for the
existence of this package. on the other hand, if a package is not used
at all, the Seastar application is not obliged to check it.

in this change, we make all the options impacting the package detection
subvars in `SeastarConfig.cmake.in`, so that Seastar application can
detect the used package properly.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

cmake/SeastarConfig.cmake.in

@@ -56,6 +56,10 @@ endif ()
 #
 
 include (SeastarDependencies)
+set (Seastar_DPDK @Seastar_DPDK@)
+set (Seastar_IO_URING @Seastar_IO_URING@)
+set (Seastar_HWLOC @Seastar_HWLOC@)
+set (Seastar_NUMA @Seastar_NUMA@)
 seastar_find_dependencies ()
 
 if (NOT TARGET Seastar::seastar)