setup.py: setuptools fallback does not link abseil when building against static libre2.a

## Summary

When building `google-re2` from source using the setuptools fallback path (i.e. without Bazel / outside of GitHub Actions), the resulting `_re2.so` extension has unresolved abseil symbols. The wheel builds successfully (shared library builds allow undefined symbols by default), but crashes at import time:

```
>>> import re2
ImportError: /path/to/re2/_re2.cpython-312-x86_64-linux-gnu.so:
undefined symbol: _ZN4absl12lts_2025051218container_internal19GetRefForEmptyClassERNS1_12CommonFieldsE
```

## Root cause

`setup.py` hardcodes `libraries=['re2']` in the Extension definition. This works when linking against a shared `libre2.so` (the dynamic linker resolves the abseil dependency chain at runtime), but when `libre2.a` is the only available library (static build via CMake with `-DBUILD_SHARED_LIBS=OFF`), the linker never sees the abseil archives that `libre2.a` depends on.

## Reproduction

1. Build re2 and abseil-cpp from source as static libraries via CMake
2. Install them to a prefix
3. Build `google-re2` from the sdist using `pip install --no-binary :all: google-re2` with `CXXFLAGS=-I/include` and `LDFLAGS=-L/lib`
4. Attempt `import re2`

## Suggested fix

Use `pkg-config --libs --static re2` to discover re2's transitive dependencies (abseil) when building from source, falling back to the current `libraries=['re2']` when pkg-config is unavailable. The CMake install of re2 already produces `re2.pc` with `Requires: absl_...`, so pkg-config can resolve the full dependency chain.

This has no effect on the Bazel build path (GitHub Actions), and is backward-compatible: when pkg-config is not installed or `re2.pc` is absent, the behaviour is unchanged.

## Context

We encountered this while building wheels for architectures where Bazel is not available (s390x, ppc64le), using CMake + setuptools as a fallback.

Start in setup.py and inspect the setuptools Extension definition and its current libraries setting. Reproduce the static CMake build with pip install --no-binary :all: and the supplied CXXFLAGS/LDFLAGS, then check how pkg-config --libs --static re2 resolves dependencies. Done means the built extension imports successfully with static libre2.a, while the existing fallback remains unchanged when pkg-config or re2.pc is unavailable.

Written by the indexing model from the issue text.