docs: def vcpkg_dir():
      """ Figure out where vcpkg is installed.
      vcpkg-exported is populated in some flavors of FB internal builds.
      C:/tools/vcpkg is the appveyor location.
      C:/open/vcpkg is my local location.
      """
      for p in ["vcpkg-exported", "C:/tools/vcpkg", "C:/open/vcpkg"]:
          if os.path.isdir(p):
              return os.path.realpath(p)
      raise Exception("cannot find vcpkg")