build: Add library-based absl detection as a fallback

This should help for cases where users can make abseil-cpp available but
wiring up the CMake-build isn't that easy (for example, while
cross-compiling).
4 jobs for ci-fix in 31 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Container
failed build-container-aarch64 #10952245
aarch64

00:00:10

passed build-container-x86_64 #10952244

00:00:29

 
  Build
skipped build-aarch64 #10952247
aarch64
skipped build-x86_64 #10952246
 
Name Stage Failure
failed
build-container-aarch64 Container
$ if [[ -z "$CI_REGISTRY_IMAGE" ]] # collapsed multi-line command
$ podman login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
/bin/sh: eval: line 136: podman: not found
Uploading artifacts for failed job
Uploading artifacts...
WARNING: container-build-report.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 127