Fix out-of-tree build of docs, fix #29757.

doc/Makefile

@@ -20,7 +20,7 @@ help:
  @echo "To fix outdated doctests, use 'make <target> doctest=fix'"
 
 
-DOCUMENTER_OPTIONS := linkcheck=$(linkcheck) doctest=$(doctest)
+DOCUMENTER_OPTIONS := linkcheck=$(linkcheck) doctest=$(doctest) buildroot=$(call cygpath_w,$(BUILDROOT))
 
 UnicodeData.txt:
  $(JLDOWNLOAD) http:https://www.unicode.org/Public/9.0.0/ucd/UnicodeData.txt

doc/make.jl

@@ -147,8 +147,12 @@ for stdlib in STDLIB_DOCS
 end
 
 const render_pdf = "pdf" in ARGS
+let r = r"buildroot=(.+)", i = findfirst(x -> occursin(r, x), ARGS)
+ global const buildroot = i === nothing ? (@__DIR__) : first(match(r, ARGS[i]).captures)
+end
+
 makedocs(
- build = joinpath(@__DIR__, "_build", (render_pdf ? "pdf" : "html"), "en"),
+ build = joinpath(buildroot, "doc", "_build", (render_pdf ? "pdf" : "html"), "en"),
  modules = [Base, Core, BuildSysImg, [Base.root_module(Base, stdlib.stdlib) for stdlib in STDLIB_DOCS]...],
  clean = true,
  doctest = ("doctest=fix" in ARGS) ? (:fix) : ("doctest=true" in ARGS) ? true : false,
@@ -171,7 +175,7 @@ makedocs(
 if "deploy" in ARGS && Sys.ARCH === :x86_64 && Sys.KERNEL === :Linux
  deploydocs(
  repo = "github.com/JuliaLang/julia.git",
- target = "_build/html/en",
+ target = joinpath(buildroot, "doc", "_build", "html", "en"),
  dirname = "en",
  devurl = "v1.1-dev",
  versions = ["v#.#", "v1.1-dev" => "v1.1-dev"]