If one sets the variable foo_DIR it will cause CMake to look for a
configuration file in that directory. In case we want to set the root
directory to an installation which doesn't have a config-mode CMake
module, and use the find module provided by ourself, we need a separate
variable that can hold the location of this directory and the most
common suffix for this seems to be _ROOT (an alternative is _PREFIX)
Allow arguments to be used on the list of dependencies, this facilitates
searching for things like e.g. Boost, and there is now only need for one
list since REQUIRED can be one of the arguments.
Modules that starts with the prefix "Use" alters the build to enable
something by default, whereas modules that only provide functionality
is not common to start with that prefix.
