Disallow install() of export() result.
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
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.
This policy was introduced in CMake version 3.3. CMake version
3.6.2 warns when the policy is not set and uses
OLD behavior. Use
cmake_policy() command to set it to
OLD behavior of a policy is
deprecated by definition
and may be removed in a future version of CMake.