Do not link MyRocks with librt on macOS (facebook#1221)

Summary:
Same patch as
https://github.com/percona/percona-server/pull/4808/files#diff-2acd82b249609d071929a6a0d083e36ef5b8807294bfe610e6552211c0c2b30e

Pull Request resolved: facebook#1221

Reviewed By: luqun

Differential Revision: D39201050

Pulled By: yoshinorim

fbshipit-source-id: 991554b

Author: laurynas-biveinis

storage/rocksdb/CMakeLists.txt

@@ -279,7 +279,10 @@ IF (HAVE_ZLIB_H)
   SET(rocksdb_static_libs ${rocksdb_static_libs} ${ZLIB_LIBRARY})
 ENDIF()
 
-SET(rocksdb_static_libs ${rocksdb_static_libs} "-lrt" "-ldl" "-lpthread")
+SET(rocksdb_static_libs ${rocksdb_static_libs} "-ldl" "-lpthread")
+IF(NOT CMAKE_SYSTEM_NAME MATCHES "Darwin")
+  SET(rocksdb_static_libs ${rocksdb_static_libs} "-lrt")
+ENDIF()
 
 MYSQL_ADD_PLUGIN(rocksdb_se ${ROCKSDB_SOURCES} STORAGE_ENGINE DEFAULT STATIC_ONLY
   LINK_LIBRARIES ${rocksdb_static_libs}