Use target_compile_definitions instead of ifdef constructions in standard GCC CMakeLists.

Description:
Switching from using ifdef constructions to target_compile_definitions for tests related to time.h will enhance code readability and maintainability. By employing target_compile_definitions, the codebase becomes more organized and easier to understand, leading to improved collaboration among developers and facilitating future modifications and updates.

User Story:
As a developer working on the time.h related tests, I want to enhance code readability and maintainability to streamline my workflow and ensure that future modifications can be implemented smoothly.