fix vpath doc builds

At present, trying to build the documentation from outside the source
tree (i.e. a "vpath build") fails. This patch fixes that. Two changes
were required:

- in a vpath build, make sure that we look in the appropriate
directory in the build tree, not just the source tree (some
SGML files are auto-generated and aren't found otherwise,
causing the build to fail)

- in a vpath build, copy over 'stylesheet.css' from the source
tree into the build tree. The generated HTML references
"stylesheet.css", so this makes that work properly.

-Neil