`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-2120311894Closes#13718
59dc9f7e69