Idiom: the vanilla way
Libraries can be built in several different "ways", for example "profiling" and "dynamic" are two ways. Each way has a short tag associated with it; "p" and "dyn" are the tags for profiling and dynamic respectively. In previous GHC build systems, the "normal" way didn't have a name, it was just always built. Now we explicitly call it the "vanilla" way and use the tag "v" to refer to it.
This means that the GhcLibWays variable, which lists the ways in which the libraries are built, must include "v" if you want the vanilla way to be built (this is included in the default setup, of course).