Add --with-llvm to the development snapshot build of Pg11

Hi,

I'm testing Postgresql 11 on debian 9.4 stable using development snapshots (stretch-pgdg-testing), and would like to test the new jit support but pg_config shows that --with-llvm was not used when building.  Any idea if that would be activated at one point ?

I've tried installing clang-3.9 and then reinstalled the same postgresql-11 binary package but from what I can see, that's not enough to get jit working in Postgres.

Thanks,

Jean-Pierre Pelletier