Summary:
this builder is used to bootstrap the ninja build tool.

On Windows and mac the manifest for ninja is set to download a pre-built executable.
While pre-built executables are available for linux they aren't portable enough
for our purposes so we need to be able to build it for ourselves.

Reviewed By: simpkins

Differential Revision: D14690992

fbshipit-source-id: b60fd02ad04f58dc7c2931280341791270609737