This allows rust-bin and rust to coexist in bulk builds (for testing, etc),
but the packages still may not be installed at the same time.
rust.mk as a solution for picking the correct rust variant was suggested
by gdt@. It is intended to be included directly by packages that do not
use cargo.mk, and indirectly by packages that do use cargo.mk.
rust.mk provides one user-settable variable:
RUST_TYPE
as before, whether to bootstrap rust from source or use
official binaries. may be "src" or "bin"
And two package-settable variables:
RUST_REQ
the minimum version of Rust required by the package.
defaults to "1.20.0"
RUST_RUNTIME
whether Rust is a runtime dependency, may be "yes" or "no"
