Skip System Package Registry in find_package() calls.

In some cases, it is not desirable to use the System Package Registry when searching for packages. If the CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY variable is enabled, all the find_package() commands will skip the System Package Registry as if they were called with the NO_CMAKE_SYSTEM_PACKAGE_REGISTRY argument.

See also Disabling the Package Registry.