BUILD_HEADLESS option with "ON" or "TRUE" value has no effect

[JosiahWI]

Summary

The source code checks for headless mode with #if BUILD_HEADLESS. This define is set by CMake to be equal to the value of the BUILD_HEADLESS CMake option, but this is incorrect. CMake options accept "ON" or "TRUE" as truthy values, but the C++ preprocessor does not. CMake should define the symbol, and it should be checked in the C++ source with #ifdef BUILD_HEADLESS.

Steps to reproduce

cmake -S . -B build_issue_24_repro -DBUILD_HEADLESS=ON -DENABLE_SOUND=FALSE
./hacking_testing/compile_proto.sh
cmake --build build_issue_24_repro
./hacking_testing/test_loop.py

When the preceding steps are executed, you should see that the window is still there with all the normal graphics, indicating that the option had no effect.