configure: add missing optional SDL2 dependency to OpenGL outdev

Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -3267,6 +3267,7 @@ libcdio_indev_deps="libcdio"
 libdc1394_indev_deps="libdc1394"
 openal_indev_deps="openal"
 opengl_outdev_deps="opengl"
+opengl_outdev_suggest="sdl2"
 oss_indev_deps_any="sys_soundcard_h"
 oss_outdev_deps_any="sys_soundcard_h"
 pulse_indev_deps="libpulse"
@@ -6903,7 +6904,8 @@ enabled zoompan_filter      && prepend avfilter_deps "swscale"
 enabled lavfi_indev         && prepend avdevice_deps "avfilter"
 
 #FIXME
-enabled sdl2_outdev     && add_cflags $(filter_out '-Dmain=SDL_main' $sdl2_cflags)
+enabled_any sdl2_outdev opengl_outdev && enabled sdl2 &&
+    add_cflags $(filter_out '-Dmain=SDL_main' $sdl2_cflags)
 
 enabled opus_decoder    && prepend avcodec_deps "swresample"