Sebastian Holtermann authored Dec 18, 2017 and Brad King committed Dec 20, 2017

In the AutogenInfo.cmake file the separator for nested lists was
`@LSEP@` which led to a speed regression because the `@` character
triggered an (unsuccessful) expression evaluation.

By setting the policy version of the CMake instance in the
`_autogen` target to 3.9, the OLD `@` evaluating behavior
controlled by policy CMP0053 is disabled.

Also the nested lists separator string is changed to `<<<S>>>`,
which solves the problem twofold.

Issue: #17570