MFC r271401:
r271401 (by asomers):

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
Sponsored by: Spectra Logic