cmake 3.5.1 + ninja not quite happy with dev-guide build system
With cmake 3.5.1 and master branch I see
CMake Warning (dev): Policy CMP0058 is not set: Ninja requires custom command byproducts to be explicit. Run "cmake --help-policy CMP0058" for policy details. Use the cmake_policy command to set the policy and suppress this warning. This project specifies custom command DEPENDS on files in the build tree that are not specified as the OUTPUT or BYPRODUCTS of any add_custom_command or add_custom_target: docs/sphinx-input/dev-manual/releng/index.rst For compatibility with versions of CMake that did not have the BYPRODUCTS option, CMake is generating phony rules for such files to convince 'ninja' to build. Project authors should add the missing BYPRODUCTS or OUTPUT options to the custom commands that produce these files. This warning is for project developers. Use -Wno-dev to suppress it.
which seems to be because in the absence of the releng repo(?) we generate a stub file at CMake time. Perhaps we can instead have a cmake target copy the stub from the source repo to the right place?
Change handling for missing releng docs
Use a placeholder document from the source tree instead of generating it
on the fly in case RELENG_PATH is not set.
Also fix other warnings that got generated with RELENG_PATH not set.