do nothing in restore_flags() if flags are not saved

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@7918 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -291,6 +291,7 @@ save_flags(){
 }
 
 restore_flags(){
+    flags_saved || return
     CFLAGS="$SAVE_CFLAGS"
     LDFLAGS="$SAVE_LDFLAGS"
     extralibs="$SAVE_extralibs"