Ignore "obsolete" warnings Emacs Lisp

This is to work around an "if-let is obsolete use if-let* instead"
warning on Emacs snapshot. My understanding is that will be resolved
and we can revert this commit, soon -- which I think would be good
because we probably want such warnings when not false alarms?

Makefile

@@ -22,7 +22,7 @@ batch-emacs := \
 byte-compile := \
   $(batch-emacs) \
   -l bytecomp \
-  --eval '(setq byte-compile-warnings t)' \
+  --eval '(setq byte-compile-warnings (quote (not obsolete)))' \
   --eval '(setq byte-compile-error-on-warn t)' \
   -f batch-byte-compile