[maybe-BUG] unison-manual.tex only with make

It costs me time to find out how to build unsion-manual.tex.

I can only build it with "make". It makes sence in a source-repository
of course.

But there should be a way or a informative error message when someone
try to build a tex-file the usual way - with a tex-environment.

"textdirectives.tex not found."
No hint about it where or what it is!

I think the simplest way would be to some tex-code where it is
\input'ed with a message like "Don't use your tex-environment.
Please use 'make' to build unison-manual.tex."

What do you think?