Add a note about required build tools and packages

Closes #518
Don't run cargo as root during `make install`

This rejiggers the dependencies for `make install` to only copy files into the
destination, never assemble anything. This adds a hard requirement that `make`
is executed before `make install`.

Closes #519
auto merge of #527 : alexcrichton/cargo/build-issues, r=brson

Related issues in the commits.