let "cabal -fSomeFlag" produce error for undeclared flags

[Oblosys]

Typos in flags can lead to confusing problems, especially with large projects that take a long time to build. A simple check could be quite useful:

~> hconfigure -fNonExistentFlag -fSomeFlag
cabal: undeclared flag 'NonExistentFlag'. Declared flags are:
  ExistentFlag, SomeFlag

[kosmikus]

There are a few things to keep in mind:

• Flag constraints can be given for arbitrary packages, not just for targets.
• It multiple targets are specified, -f flags apply to all targets.
• Not every version of a package declares the same flags.

So whatever we end up doing: a warning might be ok, an error almost certainly isn't. Even a warning has to be designed carefully so that it isn't just perceived as noise.