Provide useful error if llvm-config not in $PATH.

Previously, if llvm-config wasn't in the PATH environment variable, we'd only detect it when we tried to find libraries in a directory called "sh: llvm-config not found/lib". Now, we fail early, as soon as we've discovered that we can't find llvm-config.
Let TESLA_STRUCT_AUTOMATON reference its subject.

In order to set sensible bounds on TESLA struct automaton checking (e.g. from alloc to free), we need to be able to refer to the struct instance.