Fix system cityhash setup

cityhash does not provide any CMake packages. Add a `Findcityhash.cmake`
script to help find cityhash.

Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>

Author: ZhongRuoyu

cmake/Findcityhash.cmake

@@ -0,0 +1,26 @@
+find_path(cityhash_INCLUDE_DIR
+  NAMES city.h
+  DOC "cityhash include directory")
+mark_as_advanced(cityhash_INCLUDE_DIR)
+find_library(cityhash_LIBRARY
+  NAMES cityhash libcityhash
+  DOC "cityhash library")
+mark_as_advanced(cityhash_LIBRARY)
+
+# Unlike lz4, cityhash's version information does not seem to be available.
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(cityhash
+  REQUIRED_VARS cityhash_LIBRARY cityhash_INCLUDE_DIR)
+
+if (cityhash_FOUND)
+  set(cityhash_INCLUDE_DIRS "${cityhash_INCLUDE_DIR}")
+  set(cityhash_LIBRARIES "${cityhash_LIBRARY}")
+
+  if (NOT TARGET cityhash::cityhash)
+    add_library(cityhash::cityhash UNKNOWN IMPORTED)
+    set_target_properties(cityhash::cityhash PROPERTIES
+      IMPORTED_LOCATION "${cityhash_LIBRARY}"
+      INTERFACE_INCLUDE_DIRECTORIES "${cityhash_INCLUDE_DIR}")
+  endif ()
+endif ()