Probe, or at least hint, for the correct value of -Perl5 vs +Perl5

[audreyt]

As @geekosaur explained on #perl6 :

cabal doesn't so much try things, in the sense of try the compile and then try it again on fail. it's just a constraint solver and if you can't describe the constraint directly in the cabal file (as when you need to do configure search for perl5 guts) you can't have it automatically set that flag.

Nevertheless it might be a good idea to detect compilation failure in some way, or at least display a helpful error message so the user knows to try again with "cabal install --flags=-Perl5".