Provide (semi) sensible default search dirs

zig/sdl/hello_sdl.zig

    var dirsList = std.ArrayList([]const u8).init(gpa);
    var dirsString = std.ArrayList(u8).init(gpa);
    const downloadsDir = try std.fs.path.join(gpa, &[_][]const u8{ std.os.getenv("HOME").?, "Downloads" });
    if (std.os.getenv("SEARCH_DIRS")) |dirsEnv| {
        var dirs = std.mem.split(u8, dirsEnv, ":");
        var dir = dirs.next();
            try dirsList.append(dir.?);
        }
    } else {
        try dirsList.append("/home/luna/k/the-thing");
        try dirsList.append("/home/luna/t/zig");
        try dirsList.append(downloadsDir);
        try dirsList.append("/usr/include");
    }
    Config.searchDirectories = dirsList.toOwnedSlice();
    for (Config.searchDirectories) |dir| {
    Config.searchDirectoriesString = dirsString.toOwnedSlice();
    defer gpa.free(Config.searchDirectories);
    defer gpa.free(Config.searchDirectoriesString);
    defer gpa.free(downloadsDir);
    if (c.SDL_Init(c.SDL_INIT_VIDEO) != 0) {
        c.SDL_Log("Unable to initialize SDL: %s", c.SDL_GetError());