System-wide libtool is required even when building from a tarball produced by "make dist"

Reported by a customer: https://support.isc.org/Ticket/Display.html?id=17785

The problem is caused by a mistake in configure.ac which was inadvertently introduced by !3742 (merged). We did not catch this in our own testing because all OS images we use have the libtool package installed.

Only the development branch (9.17.3+) is affected.