Fix 13-STABLE and 14-CURRENT for real.

r564711 attempted to provide both the definitions to use the old
NO_WERROR and the new MK_WERROR. However foot-shooting protection
prevents such innovative approaches. Therefore additional logic
is required.

PR: 253000
Submitted by: cy
Reported by: cy
Approved by: jlh (maintainer, by private email)
MFH: 2021Q1