CMP0058: Ninja requires custom command byproducts to be explicit.

[xSke]

Running this with the Ninja generator results in the following warning:

[cmake] CMake Warning (dev):
[cmake]   Policy CMP0058 is not set: Ninja requires custom command byproducts to be
[cmake]   explicit.  Run "cmake --help-policy CMP0058" for policy details.  Use the
[cmake]   cmake_policy command to set the policy and suppress this warning.
[cmake] 
[cmake]   This project specifies custom command DEPENDS on files in the build tree
[cmake]   that are not specified as the OUTPUT or BYPRODUCTS of any
[cmake]   add_custom_command or add_custom_target:
[cmake] 
[cmake]    _cmrc/include/cmrc/cmrc.hpp
[cmake] 
[cmake]   For compatibility with versions of CMake that did not have the BYPRODUCTS
[cmake]   option, CMake is generating phony rules for such files to convince 'ninja'
[cmake]   to build.
[cmake] 
[cmake]   Project authors should add the missing BYPRODUCTS or OUTPUT options to the
[cmake]   custom commands that produce these files.
[cmake] This warning is for project developers.  Use -Wno-dev to suppress it.