When run
	make -C tools DESTDIR=/my/nice/dir turbostat_install
get a message
	install: cannot create regular file '/usr/bin/turbostat': Permission denied

Allow user to alter DESTDIR and PREFIX variables.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>