Fix a bug in pocl_cache
Fix a bug in pocl_cache

if POCL_CACHE_DIR was set but it wasn't yet created, pocl
would skip it and use XDG_CACHE_HOME/pocl/kcache.

Oblomov/pocl,Oblomov/pocl,franz/pocl,rjodin/pocl,rjodin/pocl,pocl/pocl,franz/pocl,rjodin/pocl,pocl/pocl,franz/pocl,Oblomov/pocl,rjodin/pocl,pocl/pocl,pocl/pocl,Oblomov/pocl,franz/pocl,franz/pocl,pocl/pocl,Oblomov/pocl