This fix moves the man page destination to the src directory, where all
its dependencies (including transitive dependencies) live. A phony `vcc`
target was also added to make the dependency graph cleaner. Since the
`vmodtool` generates 4 files from a single vcc file, an abstract target
makes things a lot easier.

Regarding `rst2man`, since the `make` build fails if you don't have it,
there's no point in treating it as a warning in `autoconf`. It's only
more complexity and confusion.