This new postgresql_server_llvmjit macro allows extensions to use
whatever server does in regards to llvmjit plugin.
Typically, the same spec file works for a postgresql with and also
without llvmjit enabled, because the byte-compiled files are
packaged based on this macro value.
The macro includes 1 if the postgresql server is compiled with
the llvmjit support and 0 if without.
Related: #1951268
7a141e8ae1