doc: Makefile: if sphinx is not found, run a check script

Right now, if the building system doesn't find Sphinx, it
bails out, without providing any instructions about what
should be done.

Instead, run a script, providing some guidance about the
steps needed for Sphinx build to work.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile

@@ -22,6 +22,8 @@ ifeq ($(HAVE_SPHINX),0)
 
 .DEFAULT:
 	$(warning The '$(SPHINXBUILD)' command was not found. Make sure you have Sphinx installed and in PATH, or set the SPHINXBUILD make variable to point to the full path of the '$(SPHINXBUILD)' executable.)
+	@echo
+	@./scripts/sphinx-pre-install
 	@echo "  SKIP    Sphinx $@ target."
 
 else # HAVE_SPHINX