docs: Completely fix the remote build tree case

My previous fix miserably failed to catch all of the invocations of
"./scripts/sphinx-pre-install", so we got build errors.  Try again with
more caffeine.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile

@@ -72,14 +72,14 @@ quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
 	$(abspath $(BUILDDIR)/$3/$4)
 
 htmldocs:
-	@./scripts/sphinx-pre-install --version-check
+	@$(srctree)/scripts/sphinx-pre-install --version-check
 	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,html,$(var),,$(var)))
 
 linkcheckdocs:
 	@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,linkcheck,$(var),,$(var)))
 
 latexdocs:
-	@./scripts/sphinx-pre-install --version-check
+	@$(srctree)/scripts/sphinx-pre-install --version-check
 	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,latex,$(var),latex,$(var)))
 
 ifeq ($(HAVE_PDFLATEX),0)
@@ -91,17 +91,17 @@ pdfdocs:
 else # HAVE_PDFLATEX
 
 pdfdocs: latexdocs
-	@./scripts/sphinx-pre-install --version-check
+	@$(srctree)/scripts/sphinx-pre-install --version-check
 	$(foreach var,$(SPHINXDIRS), $(MAKE) PDFLATEX="$(PDFLATEX)" LATEXOPTS="$(LATEXOPTS)" -C $(BUILDDIR)/$(var)/latex || exit;)
 
 endif # HAVE_PDFLATEX
 
 epubdocs:
-	@./scripts/sphinx-pre-install --version-check
+	@$(srctree)/scripts/sphinx-pre-install --version-check
 	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,epub,$(var),epub,$(var)))
 
 xmldocs:
-	@./scripts/sphinx-pre-install --version-check
+	@$(srctree)/scripts/sphinx-pre-install --version-check
 	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,xml,$(var),xml,$(var)))
 
 endif # HAVE_SPHINX