Error message is given for make distclean as generated_src is a directory

generated_src is a directory and should be removed with the -rf flag.
Similar for other directories like bin, objects, lib, html latex, objects.
Directories man, rtf, perlmod, xml have been added for convenience (when testing with other documentation formats)
Add docbook directory to be removed as well

Add docbook directory to be removed as well
Merge pull request #107 from albert-github/feature/bug_make_rm

Error message is given for make distclean as generated_src is a directory