cogl-sdl support not detected by configure

[epitron]

Cogl doesn't build with SDL support enabled by default (on my Arch system, at least).

Rig's ./configure works fine, but the make dies here:

rut.c:29:27: fatal error: cogl/cogl-sdl.h: No such file or directory
 #include <cogl/cogl-sdl.h>
                           ^
compilation terminated.

Maybe configure should add a cogl-sdl.h test?

[epitron]

And, after adding cogl-sdl support, it's not finding SDL.h:

In file included from rut.c:29:0:
/usr/include/cogl2/cogl/cogl-sdl.h:46:17: fatal error: SDL.h: No such file or directory
 #include <SDL.h>
                 ^
compilation terminated.

[epitron]

Silly hack, but this fixes it:

CFLAGS="-I/usr/include/SDL2" ./configure