Apparently a nasty trick I did in one of the selector vtable macros
was not acceptable to VS, which thinks that "string" ? NULL : NULL is
not a constant expression - it can't tell that the string literal has
a non-null value _or_ that it doesn't matter whether the value is null
or not.
Redone the vtable name construction in a way that depends only on the
actual preprocessor, not on the followup C expression semantics.
fbb979aa98