Use Makefile.rules provided with LLVM

This is currently intended as a discussion for #71.

Makefile.rules was integrated into KLEE to support older LLVM versions (<= 2.7).

I would like to use the Makefile.rules from the LLVM version against we are compiling.
This should avoid maintaining and backporting patches to the build system.

Currently it has a drawback: Optimization/Debug/Profile information cannot be overwritten to be KLEE specific.
You think it is reasonable to assume: Use the same optimization/debug/profiling information we used for building LLVM?