Adding an option to pass additional system headers to include when building Lua

[Interrupt]

When building for targets that need to add additional system headers (like Emscripten, to give Lua access to the Emscripten libC headers) the addTranslateC step does not get those headers that have been supplied by the parent build. This allows users of the library to pass an additional path of headers to include in that step.

Example usage from one of my tests:

pub fn getEmsdkSystemIncludePath(dep_sokol: *Build.Dependency) Build.LazyPath {
    const dep_emsdk = dep_sokol.builder.dependency("emsdk", .{});
    return dep_emsdk.path("upstream/emscripten/cache/sysroot/include");
}

...

const dep_zlua = b.dependency("zlua", .{
    .target = target,
    .optimize = optimize,
    .lang = .lua54,
    .additional_system_headers = getEmsdkSystemIncludePath(dep_sokol).getPath(b),
});