To follow up the thread starting at [1], since all uses of TRAD_SYNOPSIS
have been removed, and all uses of ANSI_SYNOPSIS have been renamed to
SYNOPSIS, we can now warn about the use of these.
[1] https://sourceware.org/ml/newlib/2017/msg01182.html
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>