Conditionalize build of etcupdate(8) on MK_RCS. Since etcupdate calls
merge(1), which is part of the RCS package, it must not be installed if
WITHOUT_RCS update is set. Otherwise, it will produce confusing errors.

CR:		https://reviews.freebsd.org/D691
MFC after:	1 week
Sponsored by:	Spectra Logic


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271401 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f