Fix the detection of the LLVM bitcode compiler. This is now done at KLEE
configure time, not LLVM configure time! Configure will fail without
a working LLVM bitcode compiler. The precendence of detection is as
follow:

1. Compilers set by newly added --with-llvmcc= --with-llvmcxx= configure flags.
2. Clang in LLVM build directory.
3. llvm-gcc in PATH.

This has been tested with llvm2.9 (llvm-gcc in PATH) and llvm3.3 (clang
built in LLVM build directory).