bsd.mkopt.mk: Avoid warning from makeman

Setting WITHOUT_X where X is a __REQUIRED_OPTION produces a warning.

This occurred during makeman's make showconfig invocatation, but the
warning serves no purpose there, and clutters makeman's output. Skip
the warning for the showconfig target used by makeman.

Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49123