build: untangle `CURLDEBUG` and `DEBUGBUILD` macros

build: untangle `CURLDEBUG` and `DEBUGBUILD` macros

`CURLDEBUG` is meant to enable memory tracking, but in a bunch of cases,
it was protecting debug features that were supposed to be guarded with
`DEBUGBUILD`.

Replace these uses with `DEBUGBUILD`.

This leaves `CURLDEBUG` uses solely for its intended  purpose: to enable
the memory tracking debug feature.

Also:
- autotools: rely on `DEBUGBUILD` to enable `checksrc`.
  Instead of `CURLDEBUG`, which worked in most cases because debug
  builds enable `CURLDEBUG` by default, but it's not accurate.
- include `lib/easyif.h` instead of keeping a copy of a declaration.
- add CI test jobs for the build issues discovered.

Ref: https://github.com/curl/curl/pull/13694#issuecomment-2120311894
Closes #13718