Problem:
- Modifying CXXFLAGS directly is an old CMake style.
- The giant and ever-growing list of `*_DEBUG` macros clutters the
top-level CMakeLists.txt.
Solution:
- Use the more current `add_compile_definitions` function.
- Sort all the debug options so that they are easy to view.
- Move the `*_DEBUG` macros to their own file which can be included
directly.
cef6b7b2e4
