Move definition of LLVM_LIBS up the tree.

We will need to use this variable outside of the analyser: other bits also require an explicit dependency to be declared on e.g. LLVMSupport.
Declare dependency on LLVM libs like LLVMSupport.

This should fix the order-of-libraries-passed-to-the-linker problem.
FindExecInfo finds the library, not a directory.