[Libguestfs] [PATCH] appliance: Avoid touching appliance/supermin.d directory.

This code is subtle, to say the least. When using 'supermin --build --if-newer', supermin will check if appliance/supermin.d is newer than tmp/.guestfs-UID/appliance.d, and only rebuild the full appliance if so. So we need to avoid touching appliance/supermin.d unless there's an actual change. Unfortunately simply by creating and then deleting temporary files under appliance/supermin.d we were touching appliance/supermin.d resulting in a full appliance rebuild after every 'make'. Supermin 4 actually visited every file in the supermin appliance and then every file in the full appliance and computed a checksum from them, which was slower over all but didn't suffer from this problem. Note the above only matters for developers building and running from the build directory (ie. using ./run commands). It doesn't affect libguestfs users. Rich.