Make plugin date-time optional

It is described as optional in https://gstreamer.freedesktop.org/documentation/gstreamer/gstplugin.html?gi-language=c#GstPluginDesc and is also pretty difficult to provide for Rust plugins, I feel. For C stuff the expectation is that build system would set something like GST_PACKAGE_RELEASE_DATETIME=date -u "+%Y-%m-%dT%H:%MZ" which is terrible.