If `byte-compile-dest-file-function' is non-nil, uses that
function to do the work. Otherwise, if FILENAME matches
`emacs-lisp-file-regexp' (by default, files with the extension \".el\"),
-replaces the matching part (and anything after it) with \"\.elc\";
+replaces the matching part (and anything after it) with \".elc\";
otherwise adds \".elc\"."
(if byte-compile-dest-file-function
(funcall byte-compile-dest-file-function filename)