do-release-commit-and-tag: support VPATH builds

do-release-commit-and-tag: support VPATH builds

* build-aux/do-release-commit-and-tag: Prefer $(...) to `...`.
(noteworthy): Defined earlier to factor its value.
(noteworthy_stub): New.
Use it to factor.
(help_version): Split into...
(help, version): these.
Adjust the option processing part.
Support "--option=value" in addition to "--option value".
(builddir): New.
(--builddir): New option.
* top/README-release: Document this.
Reword slightly so that the reader cannot understand that he
has to do these steps before calling do-release-commit-and-tag.