configure: error on unknown --with-sdlabi value

I accidentally tried --with-sdlabi="1.0", and it failed much later in
a weird way. Instead, throw an error if the value isn't in our
whitelist.
Signed-off-by: NCole Robinson <crobinso@redhat.com>
Message-id: 60e4822e17697d257a914df03bdb9fff4b4c0490.1462557436.git.crobinso@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

configure

@@ -2434,9 +2434,11 @@ if test $sdlabi = "2.0"; then
     sdl_config=$sdl2_config
     sdlname=sdl2
     sdlconfigname=sdl2_config
-else
+elif test $sdlabi = "1.2"; then
     sdlname=sdl
     sdlconfigname=sdl_config
+else
+    error_exit "Unknown sdlabi $sdlabi, must be 1.2 or 2.0"
 fi
 
 if test "`basename $sdl_config`" != $sdlconfigname && ! has ${sdl_config}; then