CMP0024
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Disallow include export result.

CMake 2.8.12 and lower allowed use of the include() command with the
result of the export() command.  This relies on the assumption that
the export() command has an immediate effect at configure-time during
a cmake run.  Certain properties of targets are not fully determined
until later at generate-time, such as the link language and complete
list of link libraries.  Future refactoring will change the effect of
the export() command to be executed at generate-time.  Use ALIAS
targets instead in cases where the goal is to refer to targets by
another name

The OLD behavior for this policy is to allow including the result of
an export() command.  The NEW behavior for this policy is to not to
allow including the result of an export() command.

This policy was introduced in CMake version 3.0.0.  CMake version
|release| warns when the policy is not set and uses OLD behavior.  Use
the cmake_policy command to set it to OLD or NEW explicitly.
