Honor CMake policy CMP0058

CMake policy CMP0058, introduced in CMake 3.3, requires that all intermediate files created during the build process will be declared as an output or byproduct of a target. See "cmake --help-policy CMP0058" for details.

Per this policy, CMake was giving a warning about some files generated during configuration (e.g. with configure_file) because they were files in the build directory with no apparent target command. This is not an issue since the configure will ensure that the file is always there before the build starts. Thus, we declare that we will adhere to the new policy to avoid the warning.