Fix build of POSIX file descriptor functions

Build Large File System functions for 32bit and 64bit correctly
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!
Merge pull request #65 from delcypher/detect-bitcode-compiler

Support for the detection of the LLVM bitcode compiler in the configure script.
Merge pull request #60 from MartinNowack/fix_posix

Fix build of POSIX file descriptor functions