configure: fix sdl detection using sdl2-config

If SDL2 is requested but pkg-config doesn't have a module for it
configure should fallback to use sdl*-config, but wasn't able to
because and old variable (from SDL) was being used by mistake.

Correct the variable name and complete other related changes so
there are no more references to the old SDL.

Fixes: 0015ca5c ("ui: remove support for SDL1.2 in favour of SDL2")
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190710225528.409-1-carenas@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>