Buildsystem improvements / Fix Build on macOS

I am making an effort to have redox build natively on macOS again. It seems that the build broke a while ago because nobody really uses it as a development platform on a regular basis. I intend to to be that developer that regularly builds on macOS because it is my main platform and I do most of my work there.

This first pull request fixes the kernel build: That means prefix toolchain + initfs + kernel is working. The rest of the filesystem is a different story. There are missing dependencies (autoconf etc.) and other things I have to look into. First I want to merge the basic stuff.

There are 2 additional merge requests that are part of this effort: relibc!179 (merged) cookbook!184 (merged)