autoconf: Fix libLLVM-Major-Minor-Patch.so symlink

The symlink needs to point to a relative path, so we don't break
building in a chroot.

Tested-by: Laurent Carlier <lordheavym@gmail.org>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208908 91177308-0d34-0410-b5e6-96231b3b80d8