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
Related: #1967918
Also discussed in Fedora bug #1905584
Some extensions require to build with libpq as well, but this might be
a problem for the case server uses a private libpq, because extension
would pull in another libpq.so into the same process.
By adding new sub-packages private-libs and private-devel we can make sure
only a single copy of libpq is used by server, including the extensions.
Add dependency to libpq-devel to server-devel pkg if external libpq is used
in order to let extensions not to care about what libpq the server uses.
