Static libtiff detection does not appear to link private pkg-config dependencies

### What did you do? Built Pillow from source on a Linux system where image libraries are available primarily as static archives. In this environment, `libtiff.a` is available and its pkg-config metadata is provided as `libtiff-4.pc`. The pkg-config file includes private dependencies needed for static linking, for example zstd/lzma/jpeg/zlib via `Libs.private` / `Requires.private`. A representative static pkg-config result is equivalent to: ```sh pkg-config --libs --static libtiff-4 ``` which includes transitive dependencies such as `-lzstd`, `-llzma`, `-ljpeg`, and `-lz`. ### What did you expect to happen? When Pillow chooses to build TIFF support against a static `libtiff.a`, the Pillow extension should also link the static private dependencies reported by pkg-config for `libtiff-4`. Alternatively, if those static dependencies cannot be resolved, the build should fail clearly instead of producing an extension with unresolved symbols. ### What actually happened? Pillow built successfully, but importing `PIL.Image` failed at runtime with an unresolved symbol from one of libtiff's private static dependencies: ```text ImportError: .../_imaging.cpython-312-...so: undefined symbol: ZSTD_compressStream ``` Inspection showed TIFF symbols were included in the Pillow imaging extension, but zstd symbols remained unresolved. Rebuilding the same Pillow source with TIFF disabled made the build import cleanly, which points to the static libtiff link path rather than Pillow generally. ### What are your OS, Python and Pillow versions? * OS: custom Linux distribution / source-built userland * Python: 3.12 * Pillow: 12.2.0 ### Additional context This looks like Pillow may be finding/using `libtiff.a` directly but not using `pkg-config --libs --static libtiff-4` or otherwise not honoring the private dependencies from `libtiff-4.pc`. The practical workaround was: ```sh python -m pip install /path/to/Pillow-source.tar.gz -C tiff=disable ``` but that disables TIFF support entirely. Ideally static TIFF support should either link the transitive static dependencies or fail during build with a clear diagnostic.