GHA: add abicheck

[mdavidsaver]

Add GHA usage of abicheck.

@napetrov Is there a way to define additional C preprocessor macros when abicheck dump processes headers? This library has an "expert" API which is normally hidden. User code should define PVXS_ENABLE_EXPERT_API to opt-in. I would like these added definitions to be considered public by abicheck.

I first looked for a abicheck dump -D... argument to parallel the usual -I. I then tried --gcc-options "-DPVXS_ENABLE_EXPERT_API", but saw an arg. parsing error.

[napetrov]

Thanks for trying this. You are right that -D... is the natural CLI syntax to expect here.

At the moment, abicheck dump takes the stable header-compilation context from .abicheck.yml, so --gcc-options is not accepted. The current way to enable the PVXS Expert API is:

# .ci-local/abicheck.yml
compile:
  defines:
    - PVXS_ENABLE_EXPERT_API

Then use the same configuration for both snapshots:

abicheck dump \
  lib/linux-*/libpvxs.so.* \
  --config .ci-local/abicheck.yml \
  -I "${HOME}/.cache/base-7.0/include" \
  -I "${HOME}/.cache/base-7.0/include/os/Linux" \
  -I "${HOME}/.cache/base-7.0/include/compiler/gcc" \
  -H include/ \
  -o /tmp/pvxs-CUR.json

and likewise for pvxs-PREV.json.

This does not change how PVXS is built. It only defines PVXS_ENABLE_EXPERT_API while abicheck parses the installed public headers, modeling a consumer that has opted into the Expert API. PVXS_API_BUILDING should not be defined because that is private to building the library itself.

Our experimental integration currently omitted this macro, so it covered only the ordinary consumer-visible API. I will update it to include the Expert declarations as requested.

I also agree that requiring a config file for this one-off case is not sufficiently intuitive. I will treat the missing dedicated, repeatable -D/--define option as an abicheck CLI gap. The intention is to add a narrow macro-definition option rather than restore unrestricted compiler-option forwarding, while keeping .abicheck.yml as the recommended form for reproducible CI and baseline generation.

I used -o above intentionally. Redirecting stdout to JSON is supported and useful for pipelines, but -o/--output is preferable when persisting a snapshot because abicheck owns the file write, performs atomic replacement, provides clearer write-error handling, and can support snapshot compression.