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