Added Make variables for latex, doxygen, and dot commands. Modified the...

Added Make variables for latex, doxygen, and dot commands. Modified the documentation building system to support these new variables. These changes were made in order to get helix to build documentation. References #338.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3153 41f8697f-d340-4b68-9986-7bafba869bb8