while those two changes (from DEBUG to DEBUG_$foo) are worth doing in
their own merit, I am more inclined to consider this as orthogonal
since by your own description[1] the right name to use would be _DEBUG
(with a preceding dash) and that would obviously not conflict here.

the only remaining change then would be to drop the -DDEBUG that gets
added to your BASIC_CFLAGS

Carlo
[1] https://docs.microsoft.com/en-us/cpp/build/reference/md-mt-ld-use-run-time-library?view=vs-2019