build: cmake: set more properties to alternator library

alternator headers are exposed to the target which links against it,
so let's expose them using the `target_include_directories()`.
also, `alternator` uses Seastar library and uses xxHash indirectly.
we should fix the latter by exposing the included header instead,
but for now, let's just link alternator directly to xxHash.

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

alternator/CMakeLists.txt

@@ -17,3 +17,10 @@ target_sources(alternator
     streams.cc
     ttl.cc
     ${cql_grammar_srcs})
+target_include_directories(alternator
+  PUBLIC
+    ${CMAKE_SOURCE_DIR}
+    ${CMAKE_BINARY_DIR})
+target_link_libraries(alternator
+  Seastar::seastar
+  xxHash::xxhash)