libusb: support building for emscripten

[darkvision77]

Added emscripten backend compilation.

cpp_std=c++20 is required to build with emscripten.

How to compile:

# https://github.com/mesonbuild/meson/blob/master/cross/wasm.txt
meson setup --cross-file wasm.txt build -Ddefault_library=static
meson compile -C build

[neheb]

I'm actually curious whether adding CI for emscripten is a good idea. I don't know how valuable it is.

From my experience, it blows up when needing to link to pthread.

[darkvision77]

I'm actually curious whether adding CI for emscripten is a good idea.

IMHO, CI for emscripten is a must have thing.
After all, if the original project supports builds with emscripten, then wrap should probably support them too.

[darkvision77]

@neheb, what's the problem?