bug#15616: 24.3; Setting lexical-binding in file's local variables block should trigger a warning

[Glenn Morris]

Version: 24.4

Ryan wrote:

> I don't think it is very well-known that lexical-binding as a
> file-local variable is special in this way.

Well, it's in the NEWS entry that introduces lexical-binding,
in the variable's doc, and in the elisp manual. 

> Having a warning for a setting of lexical-binding that would have no
> effect would prevent such errors.

Implemented on the first loading of a file with lexical-binding at the
end.