Add WITHOUT_UTMPX switch to the build system.

Add WITHOUT_UTMPX switch to the build system.

This knob removes the tools that are exclusively used to view and
maintain the databases maintained by utmpx, namely last, users, who,
wtmpcvt, ac, lastlogin and utxrm.

The tool w is not in this list, because it has some other functionality
which is unrelated to utmpx; it is hardlinked to the uptime tool.