Fix USE_SYSTEM_ONNXRUNTIME flag for Flatpak build

[umireon]

This pull request updates the build configuration to enable the use of system-installed ONNX Runtime and pkg-config by default, and refines how these options are handled across different platforms. It also ensures that platform-specific logic for ONNX Runtime is clearer and more robust. The most important changes are grouped below.

Default build options:

• The default values for USE_PKGCONFIG and USE_SYSTEM_ONNXRUNTIME are now set to ON in CMakeLists.txt, meaning the build will prefer pkg-config and the system ONNX Runtime unless specified otherwise.

Platform-specific ONNX Runtime handling:

• The logic in CMakeLists.txt for handling USE_SYSTEM_ONNXRUNTIME is refactored:
  • Linux: Now expects ONNX Runtime to be found via find_package, with appropriate include and link settings.
  • macOS: Always links to onnxruntime::onnxruntime, but only adds the ONNX Runtime dylib as a source if not using the system runtime.
  • Windows: Explicitly errors if USE_SYSTEM_ONNXRUNTIME is enabled, as it is unsupported.
• On non-Windows, non-macOS platforms, ONNX Runtime is only bundled and installed if USE_SYSTEM_ONNXRUNTIME is OFF.

Build presets and overrides:

• The CMakePresets.json file is updated so that all provided presets explicitly set USE_PKGCONFIG and USE_SYSTEM_ONNXRUNTIME to false, ensuring consistent behavior in CI and developer environments regardless of the new defaults. [1] [2] [3]

Minor refactoring:

• Some redundant or misplaced lines in CMakeLists.txt are cleaned up to improve readability and maintainability.

[royshil]

Looks legit