0.9.13.36: global policy / null-lexenv confusion fix

0.9.13.36: global policy / null-lexenv confusion fix
  * Do not store the global policy in null-lexenv, but include
    it in subsequent lexenvs. Fixes visibility of global policy
    in LOCALLY and MACROLET.
  * Also actually enable the SB-LDB in default build instead of
    just providing the framework to make this a good idea.
    (Accidentally left out from from 0.9.13.33)