Autogen: Fix for problematic nested lists separator
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.
Closes #17570