Error on non-existent target in get_target_property.

In CMake 2.8.12 and lower, the get_target_property() command accepted a non-existent target argument without issuing any error or warning. The result variable is set to a -NOTFOUND value.

The OLD behavior for this policy is to issue no warning and set the result variable to a -NOTFOUND value. The NEW behavior for this policy is to issue a FATAL_ERROR if the command is called with a non-existent target.

This policy was introduced in CMake version 3.0. CMake version 3.0.2 warns when the policy is not set and uses OLD behavior. Use the cmake_policy command to set it to OLD or NEW explicitly.