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 precedence of detection is as
follows:

1. Compilers set by newly added --with-llvmcc= --with-llvmcxx= configure flags.
2. Clang in LLVM build directory.
3. llvm-gcc in PATH.
4. clang in PATH.

This has been tested with llvm2.9 (llvm-gcc in PATH) and llvm3.3 (clang
built in LLVM build directory).

This addresses a major pain point for new users of KLEE who forget to
put llvm-gcc in their PATH at LLVM configure time and then are later
forced to reconfigure and rebuild LLVM just so KLEE knows the right
PATH!