[PATCH weston 1/2] os_create_anonymous_file: support XDG_RUNTIME_DIR being unset

[Bill Spitzak]

Why not have _get_runtime_dir return a static string pointer instead? It 
avoids a copy in the (common) case where XDG_RUNTIME_DIR is set, and 
only leaks memory on exit if XDG_RUNTIME_DIR is not set.