btrfs-progs: docs: placeholder for contents.rst file on older sphinx version

Older versions of sphinx require the contents.rst file otherwise the
build fails, while new versions don't need it and use index.rst.

Sphinx error:

master file btrfs-progs/Documentation/contents.rst not found
make[1]: *** [Makefile:37: man] Error 2
make: *** [Makefile:502: build-Documentation] Error 2

This build error is seen on version 1.7.6-3.

To make it work on old and new versions create a placeholder empty file
but make it a phony build target so new sphinx does not see it and
report as not in any TOC.

[ci skip]

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

.gitignore

@@ -79,5 +79,6 @@
 
 /Documentation/Makefile
 /Documentation/_build
+/Documentation/contents.rst
 
 *.patch

Documentation/Makefile.in

@@ -28,6 +28,13 @@ man5dir = $(mandir)/man5
 man8dir = $(mandir)/man8
 
 .PHONY: all man help
+.PHONY: contents.rst
+
+# Workaround for old sphinx that requires the contents.rst file
+contents.rst:
+	@if [ "$$(sphinx-build --version | cut -d' ' -f2)" \< "1.7.7" ]; then \
+		touch contents.rst; \
+	fi
 
 # Build manual pages by default
 
@@ -53,6 +60,8 @@ uninstall:
 clean:
 	$(QUIET_RM)$(RM) -rf $(BUILDDIR)/*
 	$(QUIET_RM)$(RM) -df -- $(BUILDDIR)
+	$(QUIET_RM)$(RM) -f contents.rst
+
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).