autotools: use `CURLDEBUG` to exclude TrackMemory code from unity

autotools: use `CURLDEBUG` to exclude TrackMemory code from unity

Before this patch, autotools builds excluded TrackMemory sources
(`lib/memdebug.c` and `lib/curl_multibyte.c`) based on the `DEBUGBUILD`
setting. This works in most cases because its value is the same as
`CURLDEBUG` by default, but the correct condition is `CURLDEBUG`.

It should fix `--disable-debug --enable-curldebug --enable-unity`
builds. (not tested in CI)

It also syncs behavior with cmake builds.

Ref: #16705
Closes #16723