Automatically remove source directories if they are empty.

     Change: auto-remove-source-directories
     Purpose: Automatically remove source directories if they are empty.
     Reason: In OpenPKG we use per-package %{_sourcedir} and
                     %{_specdir} definitions (macros have trailing
                     ".../%{name}"). On removal of source(s) and .spec
                     file, this per-package directory would be kept
                     (usually <prefix>/RPM/SRC/<name>/), because RPM
                     does not know about this OpenPKG convention. So,
                     let RPM try(!) to remove the two directories (if
                     they are empty) and just ignore removal failures
                     (if they are still not empty).