gnu-web-doc-update: VPATH builds

gnu-web-doc-update: VPATH builds

* build-aux/gnu-web-doc-update (--builddir): New option.
Revamp the handling of options.
Prefer $(...) to `...`.
Don't pass --tmpdir=. to mktemp, it is useless given that we specify
the template, and it is GNU mktemp specific.
Prefer set -e to long series of &&.
Restore the initial git branch, not "master".
Properly initialize submodules (don't rely only on bootstrap)
Do not reconfigure blindly, use config.status.
* top/README-release: Update instructions for gnu-web-doc-update.