test: force Autoconf config.site to /dev/null.

From https://bugs.gnu.org/76622 (automake-patches).

* t/ax/test-defs.in: Set `CONFIG_SITE' to `/dev/null' to avoid
the local system's Autoconf site defaults from breaking the test
environment. Original report from Nelson Beebe, 26 Feb 2025.
* NEWS: mention this.
2 files changed