[Cmake] ADD_CUSTOM_COMMAND etc

[Neil Killeen]

Hello again

I am endeavouring to use ADD_CUSTOM_COMMAND and ADD_CUSTOM_TARGET
so that I can build the documentation for my system.  This will
be a mixture of HTML and LaTeX source.

I have looked at the example in the FAQ, read the manual and searched
the archives, but I am still left with no clear understanding of how
this scheme functions.  I've also tried reverse engineering the Makefile
with no success.

I'll list the FAQ example and ask questions as I go: