Meson port: `-Dlocking=false` does not control dependencies fully

Unless I'm mistaken, in the autotools the --disable-locking option actually disabled everything related to locking. However, in the Meson port you still need to specify some authentication-scheme. I think I was able to disable locking and related dependencies entirely by specifying:

-Dauthentication-scheme=pwent -Dlocking=false -Dshadow=false

but that looks quite roundabout way of doing that. Unless I'm missing something, I think -Dlocking=false should disable authentication-scheme related code entirely.