Conditional compilation to avoid "assignment to free variable"

[Michael Hoffman]

I use the same .emacs file on multiple systems, and each have various 
packages installed. There are various forms in my .emacs to only 
interact with these packages if they are actually installed:

(when (locate-library "auctex")
  (load "auctex.el" nil t t)
  (setq TeX-auto-save t))

When I byte-compile, however, I get a warning like this:

emacs.el:320:9:Warning: assignment to free variable `TeX-auto-save'

Is there a way to skip over that form at compile time if the library 
cannot be found? I tried various permutations of using 
eval-when-compile, but I still get the warning.

I suppose the other solution is to wrap the setq in a boundp check, but 
this seems silly as the result of the boundp check will be the same as 
the result of locate-library.

In this case, I may be able to use a custom variable instead but I am 
looking for a more general solution.