Doxygen/Emacs Update, master branch (2019.02.28.)

Previously I added the Pango and Cairo libraries to the environment to make the dot tool (from graphviz) be able to output PNG images. But as it was discussed extensively on various ATLAS mailing lists, this broke emacs on SLC6. (Not on CentOS 7 though...)

So now I'm undoing those updates. With the assumption that we'll be using SVG images with Doxygen anyway...