If the user does not specify a DESTINATION for a target type, the
install() command checks to see if the appropriate variable from
GNUInstallDirs is set. If it is not, then it uses an appropriate
hard-coded guess.

In addition, for FILES and DIRECTORY, the user can specify a file
type instead of a DESTINATION, and the command will use the
appropriate variable from GNUInstallDirs, or a hard-coded guess if
it is not set.