Use libloading for dynamic loading

[wusyong]

libloading can load the dylib during runtime. This could be our solution for tauri-apps/tauri#3859

[Beanow]

@wusyong I've been using this approach Beanow@3290335
To avoid the pkg-config dependency on flatpak (I believe it's considered a dev dependency, so it's only available in the SDK, not runtime). This essentially uses the native dlopen function to locate it, so I can add LD_LIBRARY_PATH=/app/lib in flatpak.

BSD General Commands Manual               PKGCONF(1)

NAME
     pkgconf — a system for configuring build dependency information

I've not tried to make this work for appimages, but since we inject an APPDIR env var, wouldn't we be able to do the same with LD_LIBRARY_PATH?

Any issues you see with this approach (besides the missing error handling of course xD)

[wusyong]

@Beanow Oh I didn't know dlopen can do that.
Can you create a PR and I'll test tmr.
If everything works, I'll just update new version.

[Beanow]

For reference I'm making a PR, though it would remove the existing APPDIR stuff.
It's quite likely we can remove the need for APPDIR, but need to investigate it more.