Add --sysroot flag to CPPFLAGS, not CFLAGS

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@19405 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -1490,7 +1490,7 @@ set_default as
 if test -n "$sysroot"; then
     case "$cc_type" in
         gcc)
-            add_cflags  --sysroot="$sysroot"
+            add_cppflags --sysroot="$sysroot"
             add_ldflags --sysroot="$sysroot"
         ;;
     esac