configure.ac: Use Autoconf quadrigraphs for character set in AS_CASE

The use of square brackets for a character set collides with the use
of square brackets for m4 quote characters, so use the other quoting
mechanism that Autoconf provides, by escaping `[` as `@<:@` and so on.

Signed-off-by: Simon McVittie <smcv@collabora.com>

configure.ac

@@ -18,7 +18,7 @@ SDL_VERSION=$SDL_MAJOR_VERSION.$SDL_MINOR_VERSION.$SDL_MICRO_VERSION
 
 SDL_BINARY_AGE=`expr $SDL_MINOR_VERSION \* 100 + $SDL_MICRO_VERSION`
 AS_CASE(["$SDL_MINOR_VERSION"],
-  [*[02468]],
+  [*@<:@02468@:>@],
     dnl Stable branch, 2.24.1 -> libSDL2-2.0.so.0.2400.1
     [SDL_INTERFACE_AGE="$SDL_MICRO_VERSION"],
   [*],