Allow FFmpeg's ./configure script to use a custom sdl-config command.

Inspired from:
http://www.mail-archive.com/uclinux-dist-commits@blackfin.uclinux.org/msg01099.html

diff -u ffmpeg-0.5.2-orig/configure ffmpeg-0.5.2/configure
--- a/configure
+++ b/configure
@@ -2066,7 +2066,7 @@
 
 disable sdl_too_old
 disable sdl
-SDL_CONFIG="${cross_prefix}sdl-config"
+SDL_CONFIG="${SDL_CONFIG-${cross_prefix}sdl-config}"
 if "${SDL_CONFIG}" --version > /dev/null 2>&1; then
     sdl_cflags=`"${SDL_CONFIG}" --cflags`
     temp_cflags $sdl_cflags