Ensure CMAKE_DATA_DIR, CMAKE_DOC_DIR, and CMAKE_MAN_DIR are always
relative paths in CMake code, and set defaults accordingly. Use the
install() command instead of install_files() and install_targets().
This is more modern and also avoids stripping of the first character
from user-specified destinations.
While at it, fix the default destinations reported in the bootstrap
help.
Some of our third-party utilities have licenses that require their
copyright and license notices to be distributed with binary forms. This
commit adds installation rules to include these notices with installed
CMake documentation.