Build fixes

The main thing this fixes is a build issue related to my kernel version. I'd advise testing it on older machines because these changes might of broken builds on older kernels.

In addition the configure script now auto-detects to LLVMTOOLDIR so you are not forced to use the "Release+Asserts" build of LLVM in order to use klee-uclibc.

Once accepted (with any need modifications) we should update the KLEE documentation to instruct people to use this branch rather than the klee-uclibc tarball.