buildlink3.mk, BUILD_DEPENDS and DEPENDS

[Aleksey Cheusov <cheusov%tut.by@localhost>]

Build dependencies caused by inclusion of buildlink3.mk files do
not alter BUILD_DEPENDS variable, only DEPENDS is affected.
Can anybody explain why?

In distbb I'd like to separate install- and build- dependencies in order
to reduce amount of packages rebuild recursively. Unfortunately I cannot
do this due to this problem (relying only on DEPEND and BUILD_DEPENDS
variables). Opinions?

-- 
Best regards, Aleksey Cheusov.