doc/publican: stop excessive rebuilds

Traced down to the server/client target always rebuilding, causing a rebuild
of everything else. Rework this so the target name is a file we actually
produce and can check for a timestamp.

Note: this also changes the generated file from the doxygen directory into the
en-US publican path and renames it to (server|client)API.xml.tmp to avoid
copying it into the xml output directory.