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 extra (documenattion) directories to ignore pattern

Add extra (documenattion) directories to ignore pattern
Add docbook directory to be removed as well

Add docbook directory to be removed as well
Better message in case doxygen -u is used

In case of obsolete or not configured entries in the Doxyfile and the option -u is used a message like:
  Warning: Tag `XML_DTD' at line 1821 of file Doxyfile has become obsolete.
  To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
appears, but the item is removed from the Doxyfile.
This patch updates the message (and improves layout) to:
  Warning: Tag `XML_DTD' at line 1821 of file `Doxyfile' has become obsolete.
                  This tag has been removed.
Merge pull request #107 from albert-github/feature/bug_make_rm

Error message is given for make distclean as generated_src is a directory
Merge pull request #109 from albert-github/feature/bug_update_msg

Better message in case doxygen -u is used
Merge pull request #108 from albert-github/feature/extension_gitignore

Add extra (documenation) directories to ignore pattern
detect python2 as Python 2 binary

On some distributions like Arch Linux
"python" is Python 3 and "python2" is Python version 2.

We now try "python2" first and use it if available.
Merge pull request #110 from JonnyJD/python2

detect python2 as Python 2 binary