installing rust on a fresh instance, make sure to have a C compiler

I am following the Book in the Getting Started chapter, try my first rustc and get the error:

> error: could not exec the linker `cc`: No such file or directory (os error 2)

The manual should at least mention the need for a C compiler (or at least a linker) as a dependency if you are using rustup.sh to install the binaries.
