Is LLVM built-in?
You can run
pg_config --configure to see with which flags and options your version was compiled.
To quote the documentation:
Print the options that were given to the configure script when PostgreSQL was configured for building. This can be used to reproduce the identical configuration, or to find out with what options a binary package was built.
Is JIT enabled?
To check if JIT is enabled, examine the configuration variable named
show jit ;
Is JIT available?
Call the system information function
pg_jit_available to see if JIT can be used in this session. Returns
jit is set to
SELECT pg_jit_available() ;