Prototype buildsystem "Drake"

[Andrej Mitrovic]

Why build a tool anyway? Why not make a build library that we can use
in our build scripts?

Then we just import the library in our `build.d` script, write the
build script with the help of those library functions, and let the
user run `rdmd build.d`. No need to install any tools, just run rdmd.

I'm already using this approach for DWinProj., the only issue is that
the build script is a little large (~300 lines), but a lot of its code
could be put into a library (e.g. it has file traversing, parallel
builds, checking if all tools are present, checking if all libraries
are present, converting header files, etc..).

This would reduce the script to a few dozen lines. And that's probably
what I'll do to make my other projects easier to build.