Python 3.15, MacOS build error

This is an install on an Apple M4 chip (ARM), macOS 26.5.1 (25F80).

```
Using Python 3.15.0a6 environment at: .tox/py315
Resolved 90 packages in 2.15s
× Failed to build `google-re2==1.1.20251105`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit status: 1)

[stdout]
running bdist_wheel
running build
running build_py
creating build/lib.macosx-11.0-arm64-cpython-315/re2
copying re2/__init__.py -> build/lib.macosx-11.0-arm64-cpython-315/re2
running build_ext
building '_re2' extension
creating build/temp.macosx-11.0-arm64-cpython-315
c++ -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -O3 -arch arm64 -mmacosx-version-min=11.0 -Wno-nullability-completeness -Wno-expansion-to-defined -Wno-undef-prefix -fPIC
-I/Users/slott/.cache/uv/builds-v0/.tmpxrcAhw/include -I/Users/slott/.local/share/uv/python/cpython-3.15.0a6-macos-aarch64-none/include/python3.15 -c _re2.cc -o build/temp.macosx-11.0-arm64-cpython-315/_re2.o -fvisibility=hidden

[stderr]
/Users/slott/.cache/uv/builds-v0/.tmpxrcAhw/lib/python3.15/site-packages/setuptools/dist.py:765: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

********************************************************************************
Please consider removing the following classifiers in favor of a SPDX license expression:

License :: OSI Approved :: BSD License

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
********************************************************************************

!!
self._finalize_license_expression()
_re2.cc:15:10: fatal error: 'absl/strings/string_view.h' file not found
15 | #include "absl/strings/string_view.h"
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
error: command '/usr/bin/c++' failed with exit code 1

hint: This usually indicates a problem with the package or the build environment.
```

The needed includes are in `/opt/local/include/`

```bash
ls -l /opt/local/include/absl/strings/string_view.h
-rw-r--r--@ 1 root admin 29629 Aug 1 2024 /opt/local/include/absl/strings/string_view.h
```

Reproduce the google-re2 build with Python 3.15.0a6 on the reported Apple M4 macOS environment, then inspect _re2.cc and the extension build configuration around the missing absl/strings/string_view.h include. Done means the extension builds successfully while locating the absl headers under /opt/local/include/.

Written by the indexing model from the issue text.