Refactor CMake version handling

Move the CMake version number components out of "CMakeLists.txt" into
dedicated file "Source/CMakeVersion.cmake".  Set the TWEAK level to the
date explicitly.  Add a "Source/CMakeVersion.bash" script to update the
date, thus replacing KWSys DateStamp for CMake.  Teach the bootstrap
script to extract the version components from their new location.