When compiling with -fmudflap configure fill EXTERN_PREFIX with garbage

(some mudflap symbol), so just pick out ff_extern symbols in this case.
patch by matthieu castet, castet.matthieu free fr

Originally committed as revision 13072 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1474,7 +1474,7 @@ fi
 check_cc <<EOF || die "Symbol mangling check failed."
 int ff_extern;
 EOF
-sym=$($nm -P -g $TMPO)
+sym=$($nm -P -g $TMPO | grep ff_extern)
 extern_prefix=${sym%%ff_extern*}
 
 check_asm inline_asm '""'