Factor the CMAKE_DATA_DIR, CMAKE_DOC_DIR, and CMAKE_MAN_DIR selection
out of CMakeLists.txt and into a Source/CMakeInstallDestinations.cmake
script.  Load the script from the original location of the code.

Cache the destination values as empty strings so we know if the user
sets them explicitly.  If not, then compute defaults based on the
platform and full CMake version string.  By not caching the versioned
defaults, we can change them in a single build tree as the version
changes.

Remove duplication of the install destination defaults from the
bootstrap script.  Cache empty defaults there too.  Parse from the CMake
code the default values to report in the help output.  Keep the CMake
code in a structured format to make this reliable.