Add AG_GST_SET_PACKAGE_RELEASE_DATETIME and _DATETIME_WITH_NANO macros for configure

Add AG_GST_SET_PACKAGE_RELEASE_DATETIME and _DATETIME_WITH_NANO macros for configure

Sets GST_PACKAGE_RELEASE_DATETIME, either to the current date and time, or
to the specified datetime, or to the date of the current release based on
the .doap file.

In a GStreamer module context, one could use it like this:
AG_GST_SET_PACKAGE_RELEASE_DATETIME_WITH_NANO([$PACKAGE_VERSION_NANO],
  ["${srcdir}/$PACKAGE.doap"],
    [$PACKAGE_VERSION_MAJOR.$PACKAGE_VERSION_MINOR.$PACKAGE_VERSION_MICRO])

https://bugzilla.gnome.org/show_bug.cgi?id=623040