libmpeg2: ensure cross sdl-config is used for sdl plugin

Otherwise we end up linking against host libraries.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/multimedia/libmpeg2/libmpeg2.mk

@@ -10,6 +10,7 @@ LIBMPEG2_INSTALL_STAGING = YES
 LIBMPEG2_CONF_OPT = --without-x --disable-directx
 
 ifeq ($(BR2_PACKAGE_SDL),y)
+LIBMPEG2_CONF_ENV += ac_cv_prog_SDLCONFIG=$(STAGING_DIR)/usr/bin/sdl-config
 LIBMPEG2_CONF_OPT += --enable-sdl
 LIBMPEG2_DEPENDENCIES += sdl
 else