Bug 2959: remove SAMBAPREFIX dependency

Bug 2959: remove SAMBAPREFIX dependency

This removes the tricky SAMBAPREFIX variable which passes full-path
information from the squid build machine down to the run-time host
helper.

Such information is not always correct when crossing machines, and the
binaries being run can easily be added to PATH in the run-time host
environment instead.

The net result of doing this is removal of Samba from the build
dependencies and increased availability of the basic_smb_auth and
ext_wbinfo_group_acl helpers.