This means the build tools will no longer be built against the host
platform's libbe, which avoids compatibility problems -- e.g. an
older Haiku host libbe may not have certain features the build tools
require -- and also makes the build behave more similiar on Haiku and
other platforms. The host libroot dependency still remains and is not
easy to get rid of.
Also remove some bits of BeOS/Dano/Zeta build support.
