makeman: Use ${make} instead of plain make to compute ${requireds}

This is currently a no-op but can matter for downstreams whose ${make}
includes additional settings.

Reviewed by:	imp, emaste
Obtained from:	CheriBSD
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D41777

(cherry picked from commit 9c80d66ec1b4c5b9ac7aaf5b0fdbb1628d49c181)

tools/build/options/makeman

@@ -116,7 +116,7 @@ show()
 		exit 1
 		;;
 	esac
-	requireds=`env -i make -f ${srcdir}/share/mk/src.opts.mk \
+	requireds=`env -i ${make} -f ${srcdir}/share/mk/src.opts.mk \
 	    -V '${__REQUIRED_OPTIONS:ts,}'`
 	env -i ${make} .MAKE.MODE=normal "$@" showconfig __MAKE_CONF=/dev/null \
 	    SRCCONF=/dev/null |