New in version 3.3.
Disallow install() of export() result.
The export() command generates a file containing Imported Targets, which is suitable for use from the build directory. It is not suitable for installation because it contains absolute paths to buildsystem locations, and is particular to a single build configuration.
The install(EXPORT) generates and installs files which contain Imported Targets. These files are generated with relative paths (unless the user specifies absolute paths), and are designed for multi-configuration use. See Creating Packages for more.
CMake 3.3 no longer allows the use of the install(FILES) command with the result of the export() command.
The OLD behavior for this policy is to allow installing the result of an export() command. The NEW behavior for this policy is not to allow installing the result of an export() command.
This policy was introduced in CMake version 3.3. CMake version 3.19.0-rc3 warns when the policy is not set and uses OLD behavior. Use the cmake_policy() command to set it to OLD or NEW explicitly.
Note
The OLD behavior of a policy is deprecated by definition and may be removed in a future version of CMake.
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https://cmake.org/cmake/help/v3.19/policy/CMP0062.html