Makefile: Add DESTDIR to support cross-builds

[rprasanth]

As the present will require sudo permission,
DESTDIR will help the user change the install location as needed.
Using -D will create the destination directory, eliminating the need for a separate mkdir command.

[rprasanth]

Hi @alandekok

Thanks for merging the PR.
is there any reason for using /lib/security instead of /usr/lib/security
most of the PAM file goes to /usr right?

can we change the default to /usr/lib/security?

[mcnewton]

It depends on the system. On mine they are in /lib/x86_64-linux-gnu/security. If PAM libs are in /usr (which could be on a different filesystem) and it's not mounted for some reason then you'll have trouble logging in.