cmake: fix cares configuration
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5131bf3..187c700 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -134,7 +134,8 @@
   if(NOT CARES_ROOT_DIR)
     set(CARES_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/cares/cares)
   endif()
-  set(CARES_STATIC ON)
+  set(CARES_SHARED OFF CACHE BOOL "disable shared library")
+  set(CARES_STATIC ON CACHE BOOL "link cares statically")
   set(CARES_INCLUDE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/third_party/cares/cares")
   add_subdirectory(third_party/cares/cares)
   if(TARGET c-ares)