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.