Dispatch docs build fails if -GNinja is used

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/man/qdmanage.8.adoc
   docs/man/qdrouterd.8.adoc
   docs/man/qdstat.8.adoc

  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.

Run cmake --build "${DispatchBuildDir}" --config ${BuildType} -t docs
ninja: error: '../docs/books/user-guide/*.adoc', needed by 'docs/books/user-guide/index.html', missing and no known rule to make it
Error: Process completed with exit code 1