Make the byte-compiler warn that function OBSOLETE-NAME is obsolete.
OBSOLETE-NAME should be a function name or macro name (a symbol).
The warning will say that CURRENT-NAME should be used instead.
If CURRENT-NAME is a string, that is the `use instead' message
(it should end with a period, and not start with a capital).
WHEN should be a string indicating when the function
was first made obsolete, for example a date or a release number.
(fn OBSOLETE-NAME CURRENT-NAME &optional WHEN)