conan-provider.cmake: introduce a new policy scope, set min cmake ver…

…sion

cmake has the notion of "policy scopes" to limit the scope of the policies.
this patch introduces a module-level policy scope and sets the minimum
required cmake version to 3.24, so the cmake features depend on policy settings
such as `if(... IN_LIST ...)` behaves as expected even if the consuming project
does not declare a cmake minimum version requirement. also, this change now
enforces the version requirement so the code would fail immediately if the
cmake version is less than the minimum.

conan_provider.cmake

@@ -22,6 +22,18 @@
 
 set(CONAN_MINIMUM_VERSION 2.0.5)
 
+# Create a new policy scope and set the minimum required cmake version so the
+# features behind a policy setting like if(... IN_LIST ...) behaves as expected
+# even if the parent project does not specify a minimum cmake version before the
+# first project() call.
+# (see: https://cmake.org/cmake/help/latest/variable/CMAKE_PROJECT_TOP_LEVEL_INCLUDES.html)
+#
+# The policy-affecting calls like cmake_policy(SET...) or `cmake_minimum_required` only
+# affects the current policy scope, i.e. between the PUSH and POP in this case.
+#
+# https://cmake.org/cmake/help/book/mastering-cmake/chapter/Policies.html#the-policy-stack
+cmake_policy(PUSH)
+cmake_minimum_required(VERSION 3.24)
 
 function(detect_os OS OS_API_LEVEL OS_SDK OS_SUBSYSTEM OS_VERSION)
     # it could be cross compilation
@@ -647,3 +659,5 @@ if(NOT _cmake_program)
     get_filename_component(PATH_TO_CMAKE_BIN "${CMAKE_COMMAND}" DIRECTORY)
     set(PATH_TO_CMAKE_BIN "${PATH_TO_CMAKE_BIN}" CACHE INTERNAL "Path where the CMake executable is")
 endif()
+
+cmake_policy(POP)