The configuration variable jit determines whether JIT compilation is enabled or disabled. If it is enabled, the configuration variables jit_above_cost, jit_inline_above_cost, and jit_optimize_above_cost determine whether JIT compilation is performed for a query, and how much effort is spent doing so.
jit_provider determines which JIT implementation is used. It is rarely required to be changed. See Section 31.4.2.
For development and debugging purposes a few additional configuration parameters exist, as described in Section 19.17.
Read the upstream documentation. For corrections, first check the current version.