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.