find_package Ignoring Boost_ROOT Due to Unset CMP0074 Policy

[ugurmelihsurme]

Description
When configuring a project with CMake that depends on msgpack-cxx version 6.1.0 and Boost, CMake generates a warning indicating that policy CMP0074 is not set. As a result, CMake ignores the Boost_ROOT environment variable, potentially leading to configuration issues if Boost is installed in a custom location.

Environment

CMake Version: 3.28
msgpack-cxx Version: 6.1.0
Boost Version: 1.84.0
Operating System: Windows 10 Version (10.0.19045 Build 19045)
Compiler: MSVC 19.29.30153.0

Warning Message
CMake Warning (dev) at CMakeLists.txt:80 (FIND_PACKAGE):
Policy CMP0074 is not set: find_package uses _ROOT variables.
Run "cmake --help-policy CMP0074" for policy details. Use the cmake_policy
command to set the policy and suppress this warning.

Environment variable Boost_ROOT is set to:
C:\Users\Administrator\Documents\Workspace\Projects\XXX-3rd-party\boost

For compatibility, CMake is ignoring the variable.

Steps to Reproduce
Please follow the guidelines for creating a minimal reproducible example as outlined here: https://stackoverflow.com/help/minimal-reproducible-example

Set the Boost_ROOT environment variable to point to a custom Boost installation path.
Configure a CMake project that depends on msgpack-cxx 6.1.0 and Boost using FIND_PACKAGE(Boost REQUIRED).
Observe the warning related to policy CMP0074.

Expected Behavior
CMake should use the Boost_ROOT environment variable as a hint to find the Boost installation, without generating warnings related to policy CMP0074.

Proposed Solution
To address this warning and ensure Boost_ROOT is considered during configuration, add the following line to your CMakeLists.txt file before the call to FIND_PACKAGE(Boost):

cmake_policy(SET CMP0074 NEW)

This policy setting instructs CMake to use the _ROOT variables as hints when searching for packages, thereby respecting the Boost_ROOT variable and potentially resolving configuration issues related to locating Boost.