USE_SDL=yes -> USE_SDL=sdl (good examles everywhere)
WANT_SDL/HAVE_SDL instead of exists(localbase/lib/lib...)
See
http://www.freebsd.org/doc/en_US.ISO8859-1/books/porters-handbook/using-sdl.html
for more information on how to use USE_SDL, WANT_SDL and HAVE_SDL.
PR: follow-ups of ports/55494, ports/61877
Submitted by: Edwin Groothuis <edwin@mavetju.org>