Use a uint64_t to store encoded version numbers so we have plenty of
bits available.  Encode with room for up to 1000 minor releases between
major releases and to encode dates until the year 10000 in the patch
level.  This is necessary because CMake development versions prior to
release 2.8.0 used the date in the patch level, and this practice may be
restored after the 3.0 release.