CMake: Use built-in add_compile_definitions for *_DEBUG macros

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.

Meta/check-debug-flags.sh

@@ -11,7 +11,7 @@ while IFS= read -r FLAG; do
     # We simply search whether the CMakeLists.txt *ever* sets the flag.
     # There are (basically) no false positives, but there might be false negatives,
     # for example we intentionally don't check for commented-out lines here.
-    if ! grep -qP "set\(CMAKE_CXX_FLAGS \".* -D${FLAG}" CMakeLists.txt ; then
+    if ! grep -qP "add_compile_definitions\(\"${FLAG}" Meta/all_the_debug_macros.cmake ; then
         echo "ALL_THE_DEBUG_MACROS probably doesn't include ${FLAG}"
         MISSING_FLAGS=y
     fi