With lexical-binding t, how do I bind an external symbol dynamicall?

[Alan Mackenzie]

Hello, Emacs!

In some Elisp with no knowledge of the variable `some-flag', which will
be defined elsewhere, I want to write

        (let (some-flag) ....)

to bind `some-flag' dynamically.  The byte-compiler give me the warning:

    Unused lexical variable `some-flag'

.  Somehow, I need to tell the byte-compiler that `some-flag' will be an
external dynamic variable.  How do I do this?

I've had a look around the "Variables" page, and its sub-pages, in the
elisp manual, yet didn't find any instructions on how to get round this
problem.  Where in the manual is it described?

-- 
Alan Mackenzie (Nuremberg, Germany).