message: Use the same Android version as UE, #define ANDROID
  older Android version.
diff: SET(CMAKE_SYSTEM_NAME Android)
  -SET(CMAKE_SYSTEM_VERSION 26)
  +SET(CMAKE_SYSTEM_VERSION 21)
  SET(CMAKE_ANDROID_ARCH_ABI arm64-v8a)
  SET(CMAKE_ANDROID_NDK "$ENV{ANDROID_NDK_ROOT}")
  @@ -13,3 +13,5 @@ SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
  SET(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
  SET(HTTPLIB_USE_OPENSSL_IF_AVAILABLE OFF)
  +
  +add_definitions(-DANDROID)
  