Lower minimum CMake version to 3.22 (#5295)

* Lower minimum CMake version to 3.22

Reverts https://github.com/shader-slang/slang/pull/4193

* Update build instructions to mention older CMake versions

3 files changed, 39 insertions, 10 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8c5003401..6a87a61ab 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.25)
+cmake_minimum_required(VERSION 3.22)
 
 # Our module dir, include that now so that we can get the version automatically
 # from git describe
@@ -15,7 +15,9 @@ set(PROJECT_VERSION "${SLANG_VERSION_FULL}")
 #
 # Global CMake options
 #
-cmake_policy(SET CMP0135 OLD)
+if(${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.24")
+    cmake_policy(SET CMP0135 OLD)
+endif()
 cmake_policy(SET CMP0077 NEW)
 set(CMAKE_POLICY_DEFAULT_CMP0077 NEW)
 if(${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.25")
diff --git a/docs/building.md b/docs/building.md
index fa18ef60f..551b14491 100644
--- a/docs/building.md
+++ b/docs/building.md
@@ -9,7 +9,7 @@ version of Slang.
 
 Please install:
 
-- CMake
+- CMake (3.25 preferred, but 3.22 works[^cmake-version])
 - A C++ compiler with support for C++17. GCC, Clang and MSVC are supported
 - A CMake compatible backend, for example Visual Studio or Ninja
 
@@ -32,6 +32,8 @@ git clone https://github.com/shader-slang/slang --recursive
 
 ## Configure and build
 
+> This section assumes cmake 3.25 or greater, if you're on a lower version please see [building with an older cmake](#older-cmake)
+
 For a Ninja based build system (all platforms) run:
 ```bash
 cmake --preset default
@@ -217,3 +219,21 @@ rm -rf build # The Visual Studio generator will complain if this is left over fr
 cmake --preset vs2022 --fresh -A arm64 -DSLANG_GENERATORS_PATH=generators/bin
 cmake --build --preset release
 ```
+
+## Building with an older CMake {#older-cmake}
+
+Because older CMake versions don't support all the features we want to use in
+CMakePresets, you'll have to do without the presets. Something like the following
+
+```bash
+cmake -B build -G Ninja
+cmake --build build -j
+```
+
+## Notes
+
+[^cmake-version] below 3.25, CMake lacks the ability to mark directories as being system
+directories
+(https://cmake.org/cmake/help/latest/prop_tgt/SYSTEM.html#prop_tgt:SYSTEM),
+this leads to an inability to suppress warnings originating in the dependencies
+in `./external`, so be prepared for some additional warnings.
diff --git a/external/CMakeLists.txt b/external/CMakeLists.txt
index ebbfefb4a..4f861c759 100644
--- a/external/CMakeLists.txt
+++ b/external/CMakeLists.txt
@@ -8,6 +8,13 @@ if(NOT CMAKE_MESSAGE_LOG_LEVEL)
     set(CMAKE_MESSAGE_LOG_LEVEL NOTICE)
 endif()
 
+if(${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.25")
+    set(system SYSTEM)
+else()
+    message(STATUS "CMake 3.25 is required to suppress warnings originating in headers in external/ but you are using ${CMAKE_VERSION}, be prepared for some warnings")
+    set(system)
+endif()
+
 # Similarly, disable warnings for external projects
 if(NOT SLANG_ENABLE_EXTERNAL_COMPILER_WARNINGS)
     if(MSVC)
@@ -18,12 +25,12 @@ if(NOT SLANG_ENABLE_EXTERNAL_COMPILER_WARNINGS)
 endif()
 
 if (NOT ${SLANG_USE_SYSTEM_UNORDERED_DENSE})
-    add_subdirectory(unordered_dense EXCLUDE_FROM_ALL SYSTEM)
+    add_subdirectory(unordered_dense EXCLUDE_FROM_ALL ${system})
 endif()
 
 # Miniz
 if (NOT ${SLANG_USE_SYSTEM_MINIZ})
-    add_subdirectory(miniz EXCLUDE_FROM_ALL SYSTEM)
+    add_subdirectory(miniz EXCLUDE_FROM_ALL ${system})
     set_property(TARGET miniz PROPERTY POSITION_INDEPENDENT_CODE ON)
     # Work around https://github.com/richgel999/miniz/pull/292
     get_target_property(miniz_c_launcher miniz C_COMPILER_LAUNCHER)
@@ -36,7 +43,7 @@ endif()
 # LZ4
 if (NOT ${SLANG_USE_SYSTEM_LZ4})
     set(LZ4_BUNDLED_MODE ON)
-    add_subdirectory(lz4/build/cmake EXCLUDE_FROM_ALL SYSTEM)
+    add_subdirectory(lz4/build/cmake EXCLUDE_FROM_ALL ${system})
     if(MSVC)
 		target_compile_options(
 		    lz4_static
@@ -47,7 +54,7 @@ endif()
 
 # Vulkan headers
 if (NOT ${SLANG_USE_SYSTEM_VULKAN_HEADERS})
-    add_subdirectory(vulkan EXCLUDE_FROM_ALL SYSTEM)
+    add_subdirectory(vulkan EXCLUDE_FROM_ALL ${system})
 endif()
 
 # metal-cpp headers
@@ -56,7 +63,7 @@ target_include_directories(metal-cpp INTERFACE "${CMAKE_CURRENT_LIST_DIR}/metal-
 
 # SPIRV-Headers
 if (NOT ${SLANG_USE_SYSTEM_SPIRV_HEADERS})
-    add_subdirectory(spirv-headers EXCLUDE_FROM_ALL SYSTEM)
+    add_subdirectory(spirv-headers EXCLUDE_FROM_ALL ${system})
 endif()
 
 if(SLANG_ENABLE_SLANG_GLSLANG)
@@ -65,13 +72,13 @@ if(SLANG_ENABLE_SLANG_GLSLANG)
     set(SPIRV_WERROR OFF)
     set(SPIRV_HEADER_DIR "${CMAKE_CURRENT_LIST_DIR}/spirv-headers/")
     set(SPIRV_SKIP_TESTS ON)
-    add_subdirectory(spirv-tools EXCLUDE_FROM_ALL SYSTEM)
+    add_subdirectory(spirv-tools EXCLUDE_FROM_ALL ${system})
 
     # glslang
     set(SKIP_GLSLANG_INSTALL ON)
     set(ENABLE_OPT ON)
     set(ENABLE_PCH OFF)
-    add_subdirectory(glslang EXCLUDE_FROM_ALL SYSTEM)
+    add_subdirectory(glslang EXCLUDE_FROM_ALL ${system})
 endif()
 
 # imgui