ARROW-8280: [C++] Use c-ares_INCLUDE_DIR

Instead of c-ares::cares's INTERFACE_INCLUDE_DIRECTORIES. Because it
may not be set. c-ares_INCLUDE_DIR is always set.

Closes #6778 from kou/ci-mingw-fix-cares

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -1952,17 +1952,19 @@ endmacro()
 if(ARROW_WITH_GRPC)
   if(c-ares_SOURCE STREQUAL "AUTO")
     find_package(c-ares QUIET CONFIG)
-    if(NOT c-ares_FOUND)
+    if(c-ares_FOUND)
+      set(CARES_INCLUDE_DIR ${c-ares_INCLUDE_DIR})
+    else()
       build_cares()
     endif()
   elseif(c-ares_SOURCE STREQUAL "BUNDLED")
     build_cares()
   elseif(c-ares_SOURCE STREQUAL "SYSTEM")
     find_package(c-ares REQUIRED CONFIG)
+    set(CARES_INCLUDE_DIR ${c-ares_INCLUDE_DIR})
   endif()
 
   # TODO: Don't use global includes but rather target_include_directories
-  get_target_property(CARES_INCLUDE_DIR c-ares::cares INTERFACE_INCLUDE_DIRECTORIES)
   include_directories(SYSTEM ${CARES_INCLUDE_DIR})
 endif()