Allow --with-PROG= and --PROG-options= for arbitrary programs

As part of the llvm-general high level llvm binding, https://github.com/bscarlet/llvm-general, theres a bunch of work going on to support building using a large variety of llvm builds and install locations (in part to allow folks who for various practical reasons have multiple llvm versions installed to easily select the llvm lib install they want to link into).

this could work two different ways: provide a hook to register new programs with the programDB (and allow using those flags with any program registered in the program db), or just using those flags will add those programs to the programDB.

Ralith or BScarlet can chime in more