build: Improve wording in './configure --help'.

build: Improve wording in './configure --help'.

Reported by Benno Schulenberg <bensberg@telfort.nl> in
<https://lists.gnu.org/archive/html/bug-gettext/2025-03/msg00000.html>.

* configure.ac: Use a dummy version number named 'package', not 'dummy'.
* gettext-runtime/configure.ac: Likewise.
* gettext-runtime/intl/configure.ac: Likewise.
* gettext-tools/configure.ac: Likewise.
* gettext-tools/examples/configure.ac: Likewise.
* libtextstyle/configure.ac: Likewise.