CMP0049

Do not expand variables in target source entries.

CMake 2.8.12 and lower performed and extra layer of variable expansion when evaluating source file names:

set(a_source foo.c)
add_executable(foo \${a_source})

This was undocumented behavior.

The OLD behavior for this policy is to expand such variables when processing the target sources. The NEW behavior for this policy is to issue an error if such variables need to be expanded.

This policy was introduced in CMake version 3.0. CMake version 3.1.3 warns when the policy is not set and uses OLD behavior. Use the cmake_policy command to set it to OLD or NEW explicitly.