[CMake] cmake, latex docbook

[Cliff Yapp]

BRL-CAD's experimental CMake build is using xsltproc and fop currently
rather than TEX binaries, but the techniques probably can be adapted to
other tools - you might want to take a look at:

http://brlcad.svn.sourceforge.net/viewvc/brlcad/brlcad/branches/cmake/doc/docbook/CMakeLists.txt?revision=40516&view=markup

and subdirectories to see if that logic is helpful.  Don't know if it's
the best/right way, but it does work in testing here.

Cheers, and good luck!

CY