如何强制cmake使用新版本的CMP0077（允许从变量设置选项）
我正在尝试将 Howard Hinnant 的date库添加为我的构建的子目录。这是简单的设置：
$ git clone https://github.com/HowardHinnant/date.git
$ cat CMakeLists.txt
cmake_minimum_required(VERSION 3.18)
project(date_test)
set(BUILD_TZ_LIB ON)
set(USE_SYSTEM_TZ_DB ON)
set(ENABLE_DATE_TESTING OFF)
add_subdirectory(date)
这里没有太多事情发生。当我尝试按原样配置此构建时，使用 cmake 3.18.3，我得到一堆输出，如：
CMake Warning (dev) at date/CMakeLists.txt:30 (option):
Policy CMP0077 is not set: option() honors normal variables. Run "cmake
--help-policy CMP0077" for policy details. Use the cmake_policy command to
set the policy and suppress this warning.
For compatibility with older versions of CMake, option is clearing the
normal variable 'USE_SYSTEM_TZ_DB'.
This warning is for project developers. Use -Wno-dev to suppress it.
CMake Warning (dev) at date/CMakeLists.txt:34 (option):
Policy CMP0077 is not set: option() honors normal variables. Run "cmake
--help-policy CMP0077" for policy details. Use the cmake_policy command to
set the policy and suppress this warning.
For compatibility with older versions of CMake, option is clearing the
normal variable 'ENABLE_DATE_TESTING'.
This warning is for project developers. Use -Wno-dev to suppress it.
CMake Warning (dev) at date/CMakeLists.txt:37 (option):
Policy CMP0077 is not set: option() honors normal variables. Run "cmake
--help-policy CMP0077" for policy details. Use the cmake_policy command to
set the policy and suppress this warning.
For compatibility with older versions of CMake, option is clearing the
normal variable 'BUILD_TZ_LIB'.
This warning is for project developers. Use -Wno-dev to suppress it.
# date: USE_SYSTEM_TZ_DB OFF
# date: MANUAL_TZ_DB OFF
# date: USE_TZ_DB_IN_DOT OFF
# date: BUILD_SHARED_LIBS OFF
# date: ENABLE_DATE_TESTING OFF
# date: DISABLE_STRING_VIEW OFF
值得注意的是，我的变量被忽略了（这USE_SYSTEM_TZ_DB是OFF我想要的时候ON）。
如果我按照错误所说的去做cmake_policy(SET CMP0077 NEW)，然后添加，那么...我得到完全相同的结果。有关该策略的相同警告消息。同样忽略我设置的变量。
有没有办法设置这些变量并将它们传播到date构建中，还是我必须将它们声明为CACHE INTERNAL变量？
回答
set(CMAKE_POLICY_DEFAULT_CMP0077 NEW)在调用add_subdirectory将日期库添加到您的项目之前设置该策略的默认值可以解决问题。
根据文档，
默认情况下，命令 cmake_minimum_required(VERSION) 和 cmake_policy(VERSION) 在给定版本未设置后保留引入的策略。将 CMAKE_POLICY_DEFAULT_CMP 设置为 OLD 或 NEW 以指定策略 CMP 的默认值，其中 是策略编号。
如果CMAKE_POLICY_DEFAULT_CMP0077没有设置，则cmake_minimum_required(VERSION 3.7)在通话CMakeLists.txt的日期库的重置这一政策为原来的行为的价值。如果 CMake 没有这样做，FetchContent如果父项目更新其 CMakeLists.txt 以需要更新的版本，则通过或 git 子模块包含的子项目的构建很容易中断。