build: Remove support for a system libsmbclient

build: Remove support for a system libsmbclient

With the new --private-libraries option, there is no longer the need
to have this support, which was aimed at avoiding the duplication
between two different libsmbclient binaries in a packaged
distribution.  By using --private-libraries instead, we do not
introduce a dependency between Samba 4.0 packages and whatever other
packages are on the system.

This effectivly reverts part of
e3ffb31554927a24df35576bd753ee4eb2dc2a3c while kpeeing the improved
SAMBA_LIBRARY declaration.

Andrew Bartlett