Setting rpath on llvmjit.so?

Currently, we don't set rpath for llvmjit.so to include `llvm-config --
libdir`. That means you need to keep LD_LIBRARY_PATH set properly to
use any non-system installation of LLVM.

This isn't unique to LLVM (we only set rpath to the postgres $libdir),
so I thought it might be project policy. But I think it will be worse
for LLVM (and perhaps ICU) where people are more likely to want to run
specific versions. Comments?

Included a simple patch, but it could use some review from someone who
knows more about portable linking and our build system.

Regards,
Jeff Davis

PS: I also noticed that a autoconf run on master generates some diffs
in configure.