CMake 4「Compatibility with CMake < 3.5」エラーの直し方

# 1. 自分のプロジェクトを直す（推奨）
cmake_minimum_required(VERSION 3.10...4.4)

# 2. 依存先が直せないときは、親から下限を伝える
set(CMAKE_POLICY_VERSION_MINIMUM 3.5)

# 3. とりあえず configure を通したいだけならコマンドラインから
#    cmake -S . -B build -DCMAKE_POLICY_VERSION_MINIMUM=3.5

$ cmake -S legacy -B b_legacy_443 -G "NMake Makefiles"
# 実行結果（legacy/CMakeLists.txt は cmake_minimum_required(VERSION 3.0)）
CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.
  （中略: VERSION 引数の書き換えを促す案内）
  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
EXIT=1

$ cmake -S legacy -B b_legacy_331 -G "NMake Makefiles"
# 実行結果（同じ CMakeLists.txt を CMake 3.31.6 で configure）
CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of CMake.
-- Configuring done (0.6s)
-- Generating done (0.0s)
EXIT=0

> b_legacy_331\demo.exe
# 実行結果
demo ok

$ cmake -S dep_case -B b_dep_443 -G "NMake Makefiles"
# 実行結果（dep_case/vendor/CMakeLists.txt は cmake_minimum_required(VERSION 2.8)）
CMake Error at vendor/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.
  （中略: VERSION 引数の書き換えを促す案内）
  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
EXIT=1

$ cmake -S policy_call -B b_policy_443 -G "NMake Makefiles"
# 実行結果（policy_call/CMakeLists.txt の3行目が cmake_policy(VERSION 3.0)）
CMake Error at CMakeLists.txt:3 (cmake_policy):
  Compatibility with CMake < 3.5 has been removed from CMake.
  （中略: VERSION 引数の書き換えを促す案内）
  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
EXIT=1

cmake_minimum_required(VERSION 3.10...4.4)
project(ModernDemo C)
add_executable(demo main.c)

$ cmake -S modern -B b_modern_443 -G "NMake Makefiles"
# 実行結果（警告は1行も出ない）
-- Configuring done (0.6s)
-- Generating done (0.0s)
EXIT=0
[100%] Built target demo
EXIT(build)=0

> b_modern_443\demo.exe
# 実行結果
demo ok

$ cmake -S legacy -B b_legacy_443_min -G "NMake Makefiles" -DCMAKE_POLICY_VERSION_MINIMUM=3.5
# 実行結果（エラーが警告に変わり、configure が通る）
CMake Warning (deprecated) at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of CMake.
This warning is for project developers.  Use -Wno-author or -Wno-deprecated
to suppress it.
EXIT=0

> b_legacy_443_min\demo.exe
# 実行結果
demo ok

$ cmake -S legacy -B b_legacy_443_min34 -G "NMake Makefiles" -DCMAKE_POLICY_VERSION_MINIMUM=3.4
# 実行結果（3.4 ではエラーのまま）
CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.
EXIT=1

cmake_minimum_required(VERSION 3.10...4.4)
project(DepCase2 C)

set(CMAKE_POLICY_VERSION_MINIMUM 3.5)

add_subdirectory(vendor)
add_executable(demo main.c)
target_link_libraries(demo PRIVATE vendor)

$ cmake -S dep_case2 -B b_dep_443_set -G "NMake Makefiles"
# 実行結果（vendor の 2.8 は警告止まりで、ビルドまで通る）
CMake Warning (deprecated) at vendor/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of CMake.
This warning is for project developers.  Use -Wno-author or -Wno-deprecated
to suppress it.
EXIT=0
[50%] Built target vendor
[100%] Built target demo
EXIT(build)=0

> b_dep_443_set\demo.exe
# 実行結果
vendor ok