CMP0098

FindFLEX runs flex in directory CMAKE_CURRENT_BINARY_DIR when executing.

The module provides a FLEX_TARGET macro which generates FLEX output. In CMake 3.16 and below the macro would generate a custom command that runs flex in the current source directory. CMake 3.17 and later prefer to run it in the build directory and use CMAKE_CURRENT_BINARY_DIR as the WORKING_DIRECTORY of its add_custom_command() invocation. This ensures that any implicitly generated file is written relative to the build tree rather than the source tree, unless the generated file is provided as absolute path.

This policy provides compatibility for projects that have not been updated to expect the new behavior.

The OLD behavior for this policy is for FLEX_TARGET to use the current source directory for the WORKING_DIRECTORY and where to generate implicit files. The NEW behavior of this policy is to use the current binary directory for the WORKING_DIRECTORY relative to which implicit files are generated unless provided as absolute path.

This policy was introduced in CMake version 3.17. Use the cmake_policy() command to set it to OLD or NEW explicitly. Unlike many policies, CMake version 3.19.0-rc3 does not warn when this policy is not set and simply uses OLD behavior.