Exit if using --libc=uclibc and KLEE was not configured with uclibc
or if the configured path does not exist.

Previously if KLEE was configured and compiled without uclibc linking
would still succeed because KLEE_UCLIBC was blank so LLVM was
effectively asked to link with "/lib/libc.a" i.e. the system's native
C library!
Merge pull request #63 from delcypher/nouclibc

Exit if using --libc=uclibc and KLEE was not configured with uclibc