Update to 2.4.0.

• geoipupdate now checks that the database directory is writable. If it is not, it reports the problem and aborts.
• geoipupdate now acquires a lock when starting up to ensure only one instance may run at a time. A new option, LockFile, exists to set the file to use as a lock. By default, LockFile is the file .geoipupdate.lock in the database directory.
• geoipupdate now prints out additional information from the server when a download request results in something other than HTTP status 2xx. This provides more information when the API does not respond with a database file. In conjunction with changes to the download service itself, errors such as lacking a subscription no longer show up with the message "not a valid gzip file".
• ${datarootdir}/GeoIP is now created on make install. Reported by Antonios Karagiannis. GitHub #29.
• Previously, a variable named ERROR was used. This caused issues building on Windows. Reported by Gisle Vanem. GitHub #36.