Fix system cityhash setup

cityhash does not provide any CMake packages, so it cannot be imported
with `find_package` or used with the alias `cityhash::cityhash`.

To fix that, we need the following:
1. Use `find_library` to find the `cityhash` library.
2. When `find_library` is used, conditionally link against its result.

Signed-off-by: Ruoyu Zhong <[email protected]>

CMakeLists.txt

@@ -77,7 +77,7 @@ PROJECT (CLICKHOUSE-CLIENT)
     ENDIF ()
 
     IF (WITH_SYSTEM_CITYHASH)
-        FIND_PACKAGE(cityhash REQUIRED)
+        FIND_LIBRARY(SYSTEM_CITYHASH NAMES cityhash REQUIRED)
     ELSE ()
         INCLUDE_DIRECTORIES (contrib/cityhash/cityhash)
         SUBDIRS (contrib/cityhash/cityhash)

clickhouse/CMakeLists.txt

@@ -38,12 +38,18 @@ IF (WITH_OPENSSL)
     LIST(APPEND clickhouse-cpp-lib-src base/sslsocket.cpp)
 ENDIF ()
 
+IF (WITH_SYSTEM_CITYHASH)
+    SET (CITYHASH ${SYSTEM_CITYHASH})
+ELSE ()
+    SET (CITYHASH cityhash::cityhash)
+ENDIF ()
+
 ADD_LIBRARY (clickhouse-cpp-lib ${clickhouse-cpp-lib-src})
 SET_TARGET_PROPERTIES (clickhouse-cpp-lib PROPERTIES LINKER_LANGUAGE CXX)
 TARGET_LINK_LIBRARIES (clickhouse-cpp-lib
     absl::int128
-    cityhash::cityhash
     lz4::lz4
+    ${CITYHASH}
 )
 TARGET_INCLUDE_DIRECTORIES (clickhouse-cpp-lib
     PUBLIC ${PROJECT_SOURCE_DIR}