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

[cartazio]

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

[bscarlet]

LLVM includes a binary "llvm-config", which can tell us the rest of what we need to know about the LLVM install. We'd like to be able to specify the location of llvm-config on the command line to cabal install, as --with-llvm-config. The comments on Distribution.Simple.UserHooks(hookedPrograms) make it sound like adding a program there ought to make the flags work, but it doesn't. It'd be lovely if it did.

[cartazio]

heres the associated work that documents some of the context + cabal hackery attempts therein bscarlet/llvm-general#41

[dcoutts]

So Setup.hs does already work like this. The problem is for cabal-install, that at the point when we're doing the command line parsing we don't know the programs available. We could perhaps try allowing any (though that's tricky with GetOpt) and then validating them later.