doc: update the Doxygen configuration file.

git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libcaca/trunk@4877 92316355-f0b4-4df1-b90c-862c8a59935f
doc: update for doxygen 1.8.7.

git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libcaca/trunk@4878 92316355-f0b4-4df1-b90c-862c8a59935f