Make the default SETCAP value to be /sbin/setcap

<Dan_C> DigitalMan: minor install issue noted today - during a fresh (Linux) install on a test box, the build failed because it was unable to execute the "sudo setcap ..." command.  This was on Slackware 15.0, where the setcap executable is located in /sbin , and that directory is not in the $PATH of a normal user, or even when a normal user uses the 'sudo' command.

Also include SETCAP in the set of documented make variables.