Documentation/sphinx: skip build if user requested specific DOCBOOKS

If the user requested specific DocBooks to be built using 'make
DOCBOOKS=foo.xml htmldocs', assume no Sphinx build is desired. This
check is transitional, and can be removed once we drop the DocBook
build.

Cc: Markus Heiser <markus.heiser@darmarit.de>
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Fixes: 22cba31 ("Documentation/sphinx: add basic working Sphinx configuration and build")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile.sphinx

@@ -17,6 +17,12 @@ ifeq ($(HAVE_SPHINX),0)
 	$(warning The '$(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 "  SKIP    Sphinx $@ target."
 
+else ifneq ($(DOCBOOKS),)
+
+# Skip Sphinx build if the user explicitly requested DOCBOOKS.
+.DEFAULT:
+	@echo "  SKIP    Sphinx $@ target (DOCBOOKS specified)."
+
 else # HAVE_SPHINX
 
 # User-friendly check for rst2pdf