sdl: fix directfb driver build if host has directfb-config

Ensure our cross directfb-config is used, so the right linker paths
as passed to the compiler.

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

package/sdl/sdl.mk

@@ -6,8 +6,8 @@
 SDL_VERSION:=1.2.14
 SDL_SOURCE:=SDL-$(SDL_VERSION).tar.gz
 SDL_SITE:=http://www.libsdl.org/release
-
 SDL_INSTALL_STAGING = YES
+SDL_CONF_ENV = ac_cv_path_DIRECTFBCONFIG=$(STAGING_DIR)/usr/bin/directfb-config
 
 ifeq ($(BR2_PACKAGE_SDL_FBCON),y)
 SDL_CONF_OPT+=--enable-video-fbcon=yes