cmake: provide generic hidapi pkg-config alias

[giri256]

Provide a generic hidapi.pc package for the selected default backend while continuing to install each enabled backend's own pkg-config package.

The generic pc/hidapi.pc.in template now accepts an explicit library name and is reused by both CMake and Autotools. Backend-specific packages are generated from one parameterized pc/hidapi-backend-specific.pc.in template, including dedicated hidapi-darwin.pc and hidapi-winapi.pc packages. Autotools preserves the existing package names, descriptions, linker flags, and direct config.status pc/hidapi-*.pc outputs for compatibility with existing build systems.

Linux CI now verifies that hidapi.pc is installed and uses a small standalone Autotools consumer to compile and link through pkg-config hidapi when both hidraw and libusb backends are present.

Tests:

• CMake configure/build/install with both Linux backends, plus the standalone pkg-config consumer
• Autotools bootstrap/configure/build/install, direct config.status regeneration, and the standalone pkg-config consumer
• Synthetic Darwin, FreeBSD, Windows, and NetBSD pkg-config generation checks
• Native Windows CMake build/install with Visual Studio 2022
• Full PR CI matrix, including the new Ubuntu pkg-config compile/link step
• git diff --check and whitespace-insensitive diff audit

Fixes: #638

Assisted-by: ChatGPT:GPT-5.6-Thinking
Assisted-by: codex:gpt-5.6-sol

[Youw]

Thanks for looking into this!
I'll update this a bit per my taste, to generalize the solution.

[Youw]

Try if this works for you.