CMP0021

Fatal error on relative paths in INCLUDE_DIRECTORIES target property.

CMake 2.8.10.2 and lower allowed the INCLUDE_DIRECTORIES target property to contain relative paths. The base path for such relative entries is not well defined. CMake 2.8.12 issues a FATAL_ERROR if the INCLUDE_DIRECTORIES property contains a relative path.

The OLD behavior for this policy is not to warn about relative paths in the INCLUDE_DIRECTORIES target property. The NEW behavior for this policy is to issue a FATAL_ERROR if INCLUDE_DIRECTORIES contains a relative path.

This policy was introduced in CMake version 2.8.12. CMake version 3.15.7 warns when the policy is not set and uses OLD behavior. Use the cmake_policy() command to set it to OLD or NEW explicitly.