The only purpose of this was to use the installed version of Errors.h,
which isn't strictly needed and create some annoyance when new error
codes are added.
This allows adding new error codes to the libbe_build without breaking
the build on older Haiku versions.
Fixes the build for the newly introduced B_PARTIAL_READ and
B_PARTIAL_WRITE.
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.
