'make check' reports 'cfg: incompatible LLVM version 2.8, expected 2.8svn'

The official LLVM 2.8 release seems to work okay with Rust, yet Rust's Makefile specifically looks for 2.8svn. Doing a find/replace on the Makefile for '2.8svn' -> '2.8' works; it'd be great if the Makefile could accept 2.8 w/o having to hack it, though, or if perhaps we could force a different version of LLVM through a command-line option.
