Add support for Ubuntu named clang packages.

Ubuntu's clang packages have additional information appended to the end of
the version which causes the current clang version checker to fail.

- Building Rust v0.5 with clang v3.0-6ubuntu3 fails.
- Building Rust v0.5, incoming with clang v3.1-5ppa (backported from Debian)
works.

Closes #4441.