feat: support pkg-config across all platforms

[JyJyJcr]

Some macOS users (e.g., those using nix-darwin) rely on pkg-config to locate HDF5, but the current build script only checks Homebrew. Additionally, on Windows, if both a registry-detected version and a pkg-config version are available, the script does not allow selecting the pkg-config version.

This PR introduces a new pkg-config feature flag to explicitly enable HDF5 detection via pkg-config. The library search logic is updated to try pkg-config first on all operating systems when this feature is enabled.

For backward compatibility, this feature is enabled by default.

[mulimoen]

We will have to update all dependents of hdf5-sys to not use standard features and put pkg-config at toplevel hdf5. Unless your usecase is only using the hdf5-sys directly

[mulimoen]

Or we can always use pkg-config and fall back to other methods if pkg-config is failing. I am unfamiliar with pkg-config outside of linux, is it widely adopted?

[JyJyJcr]

At least on macOS, pkg-config is commonly used in *-sys crates. I’m less familiar with the Windows situation, but many *-sys crates simply try pkg-config first on all platforms. Though it may not cover every case, it is a simple solution.