Optimize cmMakefile::ExpandVariablesInStringNew.

We can remove the temporary allocations required for the
default-constructed t_lookup passed into the openstack by refactoring
the code slightly. Furthermore, we use a vector instead of a stack,
since the latter is based on a deque which is not required for a
heap / lifo structure.

This patch removes ~215k allocations.

This hotspot was found with heaptrack.