doc: Get stdlib dir from make system rather than executable

The `make.jl` docsystem driver was using `Sys.STDLIB` as the canonical list
and location of stdlibs. When `make.jl` is invoked as part of the make system
(e.g. `make -C doc`), I think it makes more sense to use make's idea of where
the stdlibs are. Of course, if you're using a just-built julia to create the docs,
these two notions are identical. However, the docsystem build supports (explicitly
via the `JULIA_EXECUTABLE` option), being built with an external julia executable.
In this case, we should still use the stdlib source from in tree (as we do with
the ordinary base sources). A primary motivation is to let AI agents run the
doctests before they submit PRs without having to do a whole julia build.

Author: Keno

doc/Makefile

@@ -21,9 +21,9 @@ help:
 	@echo "To fix outdated doctests, use 'make <target> doctest=fix'"
 	@echo "To run doctests using Revise (to test changes without rebuilding the sysimage), use 'make <target> doctest=true revise=true'"
 
-
+VERSDIR := v`cut -d. -f1-2 < $(JULIAHOME)/VERSION`
 DOCUMENTER_OPTIONS := linkcheck=$(linkcheck) doctest=$(doctest) buildroot=$(call cygpath_w,$(BUILDROOT)) \
-    texplatform=$(texplatform) revise=$(revise)
+    texplatform=$(texplatform) revise=$(revise) stdlibdir=$(call cygpath_w,$(build_datarootdir)/julia/stdlib/$(VERSDIR)/)
 
 UNICODE_DATA_VERSION=13.0.0
 $(SRCCACHE)/UnicodeData-$(UNICODE_DATA_VERSION).txt:

doc/make.jl

@@ -1,5 +1,24 @@
+# Get the buildroot and stdlibdir from the make environment to make sure we're
+# generating docs for the current julia source tree, regardless of what julia
+# executable we're using. If these arguments are not passed, fall back to
+# assuming that we're running a just-built version of julia and generating docs
+# in tree.
 let r = r"buildroot=(.+)", i = findfirst(x -> occursin(r, x), ARGS)
-    global const buildrootdoc = i === nothing ? (@__DIR__) : joinpath(first(match(r, ARGS[i]).captures), "doc")
+    if i === nothing
+        global const buildrootdoc = @__DIR__
+        global const buildroot = abspath(joinpath(buildrootdoc, ".."))
+    else
+        global const buildroot = first(match(r, ARGS[i]).captures)
+        global const buildrootdoc = joinpath(buildroot, "doc")
+    end
+end
+
+let r = r"stdlibdir=(.+)", i = findfirst(x -> occursin(r, x), ARGS)
+    if i === nothing
+        global const STDLIB_DIR = Sys.STDLIB
+    else
+        global const STDLIB_DIR = first(match(r, ARGS[i]).captures)
+    end
 end
 
 # Install dependencies needed to build the documentation.
@@ -28,7 +47,6 @@ cp_q(src, dest) = isfile(dest) || cp(src, dest)
 
 # make links for stdlib package docs, this is needed until #552 in Documenter.jl is finished
 const STDLIB_DOCS = []
-const STDLIB_DIR = Sys.STDLIB
 const EXT_STDLIB_DOCS = ["Pkg"]
 cd(joinpath(buildrootdoc, "src")) do
     Base.rm("stdlib"; recursive=true, force=true)