Add `when` clauses to variables (WIP)

[Paul-Ferrell]

Adding a _when clause to any complex variables.

mytest:
  variables:
    input?:
      - 
      cc: ""
      cxx: ""
      threads: 1

 
    input:
      - _when: "sys_name == fireweasel"
        cc: cc
        cxx: CC
      - _when: "sys_name == iceferret"
        cc: cpp
        cxx: CPP

• Each matching _when item is appended.