Idea: move Emscripten include path injection and depending on emscripten sdk into build.zig

[floooh]

...we're iterating over the C library dependencies here anyway:

https://github.com/floooh/sokol-zig/blob/d9f3ef983b7021417cc91bd5788cd925ca06aa83/build.zig#L351-L367

...maybe the Emscripten include path could be injected there, as well as depending on the Emscripten SDK setup step? That way the toplevel project wouldn't need to take care of that, and it's needed for every C library anyway.