How to tell if Postgres was built with the option to include LLVM technology for JIT?

Postgres 11 gained the ability to perform Just-in-Time Compilation (JIT) based on LLVM technology. The manual notes that this feature depends on Postgres having been built with the --with-llvm option. ➥ How can one verify their build of Postgres includes LLVM? Assuming LLVM is installed… In Postgres 11, JIT is disabled by default. In Postgres 12, the feature is enabled. That raises the question: ➥ How can one tell if JIT is currently enabled or disabled?