Extended Description

Build times have increased after the tools were changed to use llvm-config to
generate the list of libraries to link with. Here's the scenario:

1. llvm-config depends on all libraries so a change to any one of them results
   in llvm-config being rebuilt.

2. All tools depend on llvm-config so any change to it results in all tools being
   rebuilt.

For example, consider a change to lib/Transforms/Scalar. It should only affect
opt, gccas, gccld, and maybe llc and lli. However, when you rebuild, all tools
are relinked. There's no reason that llvm-dis, llvm-as, etc. need to be
relinked. Previously the makefiles did not re-link these tools for which a
dependency was not changed.

Its unclear what the correct solution is. We need an "existence" dependency for
the tools on llvm-config.