Merge ebuilds to reduce maintenance costs.

Currently rust-0.8.ebuild and rust-9999.ebuild have some minor
differences. After this change, you'll modify rust-9999.ebuild to fix
build problems in master as usual, and then all you need to support a
new release is just `cp rust-9999.ebuild rust-x.y.ebuild; <edit KEYWORDS
in rust-x.y.ebuild>`.

I don't know why you've avoided using econf, so I left the line
"${ECONF_SOURCE:-.}"/configure.