By convention, this prebuilt directory is where Rebol executables are kept
which are able to do the configuration and C code generation to do a
bootstrap build.
Important to realize about the bootstrapping strategy is that there are only two builds that are supposed to be "guaranteed" to work to build the code in any moment: the "stable" bootstrap build, and the current commit. So while that although you can build the current codebase with itself, it might stop working at any point. This is because the "shim" which is used to ensure the build tools run only targets one version.
The bootstrap commit is currently at git commit short hash: 8994d23
At one point the executables were committed directly into the Git repository. This was decided to be undesirable. So instead, the files are now stored on an S3 instance. Having the build process get these files automatically is a goal, but in the meantime, these are the current links:
LINUX: https://s3.amazonaws.com/r3bootstraps/r3-linux-x64-8994d23
WINDOWS: https://s3.amazonaws.com/r3bootstraps/r3-windows-x86-8994d23.exe
OS X: https://s3.amazonaws.com/r3bootstraps/r3-osx-x64-8994d23-debug (Note: There was some reason the release build that had been generated for OS X wasn't used--it was compiled with unwanted dependencies, or somesuch. So the bootstrap build that was picked has debug instrumentation in it.)
ANDROID: https://s3.amazonaws.com/r3bootstraps/r3-android-arm-8994d23
OPENBSD: (submitted by Stéphane Aulery) https://s3.amazonaws.com/r3bootstraps/r3-openbsd-x64-8994d23