Check for starlark_doc_extract instead of Bazel version.

See bazel-contrib/bazel_features#34 (comment).

docs/BUILD

@@ -17,7 +17,7 @@ load("@pip_deps//:requirements.bzl", "requirement")
 load("@rules_python//python:defs.bzl", "py_binary")
 load("@rules_python//python:proto.bzl", "py_proto_library")
 load("//elisp:defs.bzl", "elisp_binary", "elisp_manual")
-load("//private:defs.bzl", "bazel_is_at_least", "merged_manual")
+load("//private:defs.bzl", "DOCUMENT_EXTENSIONS", "merged_manual")
 
 package(
     default_applicable_licenses = ["//:license"],
@@ -41,8 +41,6 @@ genrule(
     cmd = "makeinfo --no-split --output=$@ -- $<",
 )
 
-DOCUMENT_EXTENSIONS = bazel_is_at_least("7")
-
 DOCS = [
     "elisp",
     "emacs",

private/defs.bzl

@@ -23,6 +23,8 @@ load(":generated.bzl", "CHR", "ORD")
 
 visibility(["//docs", "//elisp", "//elisp/proto", "//emacs", "//tests/wrap"])
 
+DOCUMENT_EXTENSIONS = hasattr(native, "starlark_doc_extract")
+
 def bazel_is_at_least(version):
     """Return `True` if the Bazel version is at least <var>version</var>."""
     return versions.is_at_least(version, BAZEL_VERSION)