install: Add sane set of defaults for DESTINATION and file type parameters
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.