Partially fix python detection for running llvm-lit.

The problem is newer LLVM versions (e.g. 3.3) detect python at
their configure time whereas older versions don't (i.e. 2.9). So
I don't want to add python detection to KLEE's configure if LLVM
already does the work for us. We need to move off llvm 2.9 anyway.
Only run klee-uclibc tests if KLEE was configured with klee-uclibc
support.