[PATCH] docs: Prefer 'python3' when building htmldocs

[Victor Erminpour]

Prefer 'python3' and 'sphinx-build-3' when building htmldocs.
Building htmldocs fails on systems that don't have 'python'
and 'sphinx-build' installed, but do have 'python3' and
'sphinx-build-3' available.

Signed-off-by: Victor Erminpour <victor.erminpour@xxxxxxxxxx>
---
 Documentation/Makefile | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/Documentation/Makefile b/Documentation/Makefile
index d77bb607aea4..00c400523e15 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -11,6 +11,7 @@ endif
 
 # You can set these variables from the command line.
 SPHINXBUILD   = sphinx-build
+SPHINXBUILD3  = sphinx-build-3
 SPHINXOPTS    =
 SPHINXDIRS    = .
 _SPHINXDIRS   = $(patsubst $(srctree)/Documentation/%/index.rst,%,$(wildcard $(srctree)/Documentation/*/index.rst))
@@ -61,11 +62,23 @@ loop_cmd = $(echo-cmd) $(cmd_$(1)) || exit;
 # $5 reST source folder relative to $(srctree)/$(src),
 #    e.g. "media" for the linux-tv book-set at ./Documentation/media
 
+HAVE_PYTHON3 := $(shell if which $(PYTHON3) >/dev/null 2>&1; then echo 1; else echo 0; fi)
+HAVE_SPHINX3 := $(shell if which $(SPHINXBUILD3) >/dev/null 2>&1; then echo 1; else echo 0; fi)
+PYTHON_BIN = $(PYTHON)
+
+# If we have both python3 and sphinx-build-3,
+# prefer python3 over python.
+ifeq ($(HAVE_PYTHON3),1)
+    ifeq ($(HAVE_SPHINX3),1)
+        PYTHON_BIN = $(PYTHON3)
+    endif
+endif
+
 quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
       cmd_sphinx = $(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/media $2 && \
 	PYTHONDONTWRITEBYTECODE=1 \
 	BUILDDIR=$(abspath $(BUILDDIR)) SPHINX_CONF=$(abspath $(srctree)/$(src)/$5/$(SPHINX_CONF)) \
-	$(PYTHON) $(srctree)/scripts/jobserver-exec \
+	$(PYTHON_BIN) $(srctree)/scripts/jobserver-exec \
 	$(SHELL) $(srctree)/Documentation/sphinx/parallel-wrapper.sh \
 	$(SPHINXBUILD) \
 	-b $2 \