Use apt for clang

[joepvd]

Currently, clang is installed from a tarball, which gets put in a surprising place. The PATH variable needs to point to /usr/local/clang-3.9.0/bin for this to take effect, and this path is higher in priority than /user/bin.

This means that any attempts to change clang will be moot, if one is not aware of the PATH setup.

It might be a better approach to switch to the LLVM PPA in order to have behavior that is more predictable for end users. See here for an example of confusion.